The carbon tax is currently 14.31cents per litre, that’s about 10%. It’s an incentive. To fully wipe out that cost, you don’t need to buy an EV, you could drive 10% less, or buy an ICE vehicle that is 10% more efficient (or some combination). That’s very easy to do in a country where most of us drive large vehicles, and make too many un-combined trips. Drop one trip in 10, or combine it with one of the other 9 and you get to spend your rebate money on beer instead of gasoline.
Subsidies and special taxes are super in-efficient. Besides requiring a whole slew of bureaucracy to administer it, it never applies to everything fairly. That tax you suggest on new ICE vehicles doesn’t dissuade anyone from parking their jacked up f150 one day a week, and it doesn’t reward the person who buys a used car for their commute instead of a used SUV. All those little decisions get incentivized, and they allow people to make their own decisions about how to pollute less, instead of doing the 1 thing some government has decided to be the official, subsidized solution.