Who thinks that multiple countries will end up imposing a carbon tax on crytocurrency owners & miners? If this is done, what would happen to the price of BTC? Do you think bitcoin would make changes to adapt (like ending mining sooner than 2140)?
Electric car owners use electricity. Do they pay for the carbon taxes? No, they get benefits for using "clean energy".
Would bitcoin miners have to pay carbon taxes for their consumption? Isn't it still electricity?
Electric companies should be (and are!) required to pay for their carbon footprint and that should translate into the price of the electricity. Higher price for electricity may make mining no longer profitable there and get relocated.
This is the correct and realistic path. Then you'll see how many miners actually work with cleaner energy than you use.
On the other hand, if you ask about carbon tax for crypto owners it means that you have no idea what you are talking about and you should read more before opening your mouth.
Would Bitcoin be secure with fewer miners?
Yes. I don't know the numbers, but it would clearly be still secure even with less miners. But as long as it's profitable, I don't see them give up.