It crops up in comments from electric-car advocates and clean-energy users fairly regularly.

And it goes something like this: "Big Oil and Gas are on the verge of collapse, as clean energy and electric cars make them obsolete."

Occasionally the sentiment is followed by an approving emoji.

DON'T MISS: Electric cars pose little threat to oil demand (Financial Times, login may be required)

With large-scale installations of clean generating capacity now reportedly no more expensive—in some cases—than the most efficient fossil-fueled plants, the electric-power horizon is changing rapidly.

But electric cars leading to a wholesale global collapse in demand for gasoline within the next few years?

Stop with that already, folks.

BMW i3 and Volkswagen e-Golf electric cars using Combined Charging System (CCS) DC fast charging

BMW i3 and Volkswagen e-Golf electric cars using Combined Charging System (CCS) DC fast charging

Or at least add an asterisk that points to a note saying "in a few decades, maybe."

Plug-in electric cars have many appealing features: they're nicer to drive, they're cheaper per mile to operate than the average conventional car in almost all U.S. locations, and they are partially or fully zero-emission.

They are still notably pricier to buy than conventional cars of the same size, however, though the steady 7-percent annual cost reduction in lithium-ion cell prices will steadily chew away at that problem until it's eliminated.

READ THIS: How Much And How Fast Will Electric-Car Battery Costs Fall?

That said, the world has 1.2 billion vehicles on its roads, roughly, and after six years, there are fewer than 2 million vehicles with plugs.

The oil industry is quite confident that it has decades of demand yet to fuel those vehicles—which bring mobility, economic opportunity, and freedom to their owners and users.

So here are the reasons that electric cars aren't threatening the imminent collapse of Big Oil. There are actually at least three, followed by a couple of points to keep watch on.

Gas pump

Gas pump

(1) Fuel-economy requirements and carbon-emission limits

Whether you call them fuel-economy rules or limits on vehicular carbon emissions, the effect is the same: every new vehicle sold uses less fuel.

Miles driven in mature markets (North America, Europe) aren't notably rising, which means that gasoline consumption is already declining slowly.

In fact, U.S. gasoline consumption peaked in 2006 and fell each year for nine years thereafter. It rose slightly last year for the first time in a decade.

Yes, China is seeing many more cars on the road, and growth areas like India and South America will see more vehicles on the road as well.

Indian traffic [Image: Flickr user Peter Eich]

Indian traffic [Image: Flickr user Peter Eich]

But many of them already have aggressive rules for fuel economy and, in the case of China, electrification.

Whether they are able to follow through in areas where mass-market vehicles cost half what they do in North America, however, remains to be seen.

Right now, global gasoline consumption is expected to continue rising.

CHECK OUT: Gasoline demand rises in U.S., China, and India; more growth expected (May 2016)

(2) The transition will be slow

Remember those 1.2 billion vehicles on the world's roads?

Even with global sales approaching 100 million new vehicles a year, making a significant dent in the global fleet takes many, many years.

Consider the case of catalytic converters to reduce emissions of carbon monoxide, hydrocarbons, and nitrogen oxides.

catalytic converter

catalytic converter

They were first introduced in the U.S. on 1975 model-year cars. They didn't become common in Europe for another 10 years, and the rest of the world followed in the 1990s and 2000s.

In other words, it took 30 years for the technology to become standard—and far longer for it to spread into a majority of those 1.2 billion vehicles.

(3) An entirely new and costly industry has to be built

That 7-percent annual cost reduction metric for lithium-ion cells is based on more than 20 years of data from consumer-format cells.

But to move even a substantial minority of the world's 100 million vehicles to partial or full electric power will require an entirely new network of global lithium-ion cell production.

Oil well (photo by John Hill)

Oil well (photo by John Hill)

Remember, a mobile phone has one lithium-ion cell; a laptop may have four to eight. A Tesla uses between 5,000 and 10,000 of them.

That's why Tesla Motors CEO Elon Musk, several years ago, identified the need for the company to build a "gigafactory" that would produce cells and battery packs at a scale that hadn't been seen anywhere in the world.

To put large battery packs, with ranges of 50 miles (for plug-in hybrids) to 300 miles (for battery-electrics), into even one-fifth of those 100 million vehicles will require hundreds or thousand of gigafactories.

With a cost of $2 billion to $5 billion, that ain't chump change.

It will come, if the demand is there, but automakers are understandably cautious about signing contracts for cell purchases for vehicles whose demand thus far remains uncertain.

Tour of Tesla battery gigafactory for invited owners, Reno, Nevada, July 2016

Tour of Tesla battery gigafactory for invited owners, Reno, Nevada, July 2016

(4) However, cell-cost reductions can't be escaped

A "hockey-stick" rise in consumer demand for plug-in electric cars may not arise for several more years.

Again, it will require the cars to cost within 10 or 20 percent of a comparable vehicle with a gasoline engine alone.

Gas prices seem to matter—they're currently low, and projected to stay that way for some time, absent geopolitical surprises—as do the hurdles posed by franchised auto dealerships and inept marketing by major automakers.

There's no indication, however, that lithium-ion cell costs won't continue to fall.

So when cells fall below $100 per kilowatt-hour, meaning a 60-kwh battery pack would cost $5,000 or less, the picture begins to change.

Tour of Tesla battery gigafactory for invited owners, Reno, Nevada, July 2016

Tour of Tesla battery gigafactory for invited owners, Reno, Nevada, July 2016

And it probably continues to change rapidly.

That said, it'll be the end of many of our lifetimes before global gasoline demand has not only leveled off but plummeted.

(5) And here's one to watch for

Remember when smoking was possible pretty much everywhere?

That included airplanes, restaurants, courtrooms, offices, cars, and any place else you might spend time.

In only 30 years or so, smoking has moved from something accepted without questioning to something that is strictly regulated because of its adverse effects on the non-smokers exposed to it.

Smoking

Smoking

That leads us to ask when the same principle will begin to be applied to motor vehicles.

When will vehicles with tailpipes that emit climate-change gases and other substances start to be viewed as morally wrong?

If you see that discussion starting to percolate and spread, we'd suggest that's a good indicator that it may presage a (slow) decline in gasoline consumption.

CONSIDER THIS: When Will We Start To See 'Tailpipes' On Cars As Morally Wrong?

Though we suspect, sadly, that it may take many years yet.

As civil-rights activists have noted, the path to social change is long, winding, and filled with setbacks.

You have been warned.

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