This is a popular idea, I think people are a bit deluded by the fact that Wind+Solar got so cheap. Don't get me wrong: This is great. We'll likely be able to get rid of fossil-based electricity simply due to costs (even though it's too slow).
But it's simply not plausible that you will find a cheaper technology in every area.
Finding a way to fly a plane without carbon emissions is a challenge to begin with. Doing it cheaper than with kerosene - there isn't even a faint of an idea how to do that.
In some areas like cement the only option seems to be carbon capture and storage. There's no way whatsoever that a cement plant + a ccs plant is cheaper than a cement plant alone.
The reasoning behind the carbon tax is simply that every participant can make a decision for themselves. "Do I need CO2 emissions to run my business?" "Does a different business have a greater need to emit CO2 emissions?" This is the type of thinking one is trying to encourage with the introduction of a CO2 tax.
Sure, maybe you can't make efficient electric planes. But maybe you can replace 90% of short range flights with trains and make electric planes unnecessary.
That is actually a misconception. There is no such threshold, at least not for carbon dioxide (it's a bit more complicated for methane and some other greenhouse gases). It accumulates in the atmosphere.
There is some uptake of co2 by natural processes, but that's roughly relative to the amount you put in. If you put in more co2 the concentration of co2 in the atmosphere will keep growing.
Yet, improving the cement plant and the carbon capture one makes it much easier for a government to intervene and require zero emissions.
We'll get aviation and steel production to use synthetic fuels in due time, and we will have plastics produced from captured CO2. Yes, those are more expensive, but nobody is saying that the solution should be left entirely to the market. But neither of them make any sense right now, and that's because the technology isn't there.