Eventually combustion engines WILL be illegal. Diesels will naturally be the first to go, in fact I expect they'll be taxed out of the market shortly due to their noxious nitrogen output.
As petroleum production shrinks over years (which from an environmental point of view it's sure to under legislation) then it will become more expensive. That in turn will take care of the "classic" market; those who can afford to run them will continue to run/fuel them. The loss of the petrol market is probably many years away though, the replacement technologies will be more than adequate by them.
My worry when I look at the US is that large infrastructure development is ongoing in order to feed their oil deficit... with such recent investment the oil giants there are unlikely to be accepting any wind-down plan soon. If they ever would, of course