If we assume that all manufacturers make a u-turn in the next 5 years, and also jump on board, then 15 years out we're looking at maybe 5%. In no way will it be "largely killed off" in that time period.
First I'm clearly being way too wishful - really just saying what
could realistically happen.

So in that spirit then, let's say you effectively can't buy a new gas car in 5 yrs, shouldn't 10 years be enuf for the existing ones to fade away? The other large unpredictable factor though is people won't
want one anymore, for many reasons, including that the alternatives are far too superior, cheaper to use, etc. The same way film cameras went away -
fast.
Our addiction to fossil fuels is what's known as a
wicked problem; BY FAR the largest hurdle to solving our addiction is the addicts themselves... the technology is available AND affordable. People just need to ACT. Stop with the procrastination, the self-justifican, the excuses and ACT. 100% of the people I work with could quit fossil fuels in less than 1 year. EVERY... ONE... OF... THEM... but they won't... To be generous they're simply pathetic. They ALL make ~$100k+/yr. It's sad.... just REALLY... REALLY sad.... .
I also think though that most people don't care what powers their car and they'll gladly switch to whatever if they judge it to be better - this is where the magic of Tesla comes in. But yes in general, people often won't do the right things, be that for their personal health, or that of other things such as planets. So governments are also the real problem because they do the same things as individuals. Just like there isn't full funding to properly survey the sky, or develop plans for preventing, smaller asteroids/comets that could impact earth. What is more shortsighted or irresponsible as a species than that...