High pressure hydrogen is a thing too and regular petrol engines can be converted to run on it. Toyota already did it with an old corolla and a rally Yaris https://mag.toyota.co.uk/restomod-ae86/ https://mag.toyota.co.uk/hydrogen-gr-yaris-showcases-experimental-engine/ so not fully shutting the door but still a big hindrance.

If left unchecked yes, but if it’s someone’s pet then it won’t be unchecked. I still believe that all pet cats should be neutered (until we have the stray population down anyway) but if they have been neutered then you shouldn’t force them to live their lives indoors (again if you’re in the natural ranges for the wildcats they descended from). Cats have extremely large territorial ranges so unless you live in a mansion your cat will not have enough space if they are stuck indoors 24/7.

There is no difference between a domestic cat and a wildcat, as in they’re able to interbreed because they’re genetically the same species. So if you’re in a region where they would naturally exist (effectively all of Eurasia) then it’s perfectly fine to let your cats go outside.

TLDR: It’s only bad to let your cats out if you live in the Americas or Australia.