Colouring is a gimmick and should only ever be seen in that way. I enjoy doing them and take pains to get them as accurate as possible.... where its possible (only yesterday I googled a 1960s sewing machine to get the correct colour!). People that request them seem to enjoy the process so in that way, its a win win. Never discard the original, would always be my advice. It can never be replaced and also, who knows what software is around the corner to further blow our minds?
At the moment, AI colouring is complete tosh in my opinion, but who knows what the future holds?
Remini can be a useful tool in small doses but because of how it works, its over-use is worrying.
Having said all that, I often take photos and desaturate them to B/W because for some pics, you can't beat the impact.
As for colouring pics without permission, well as far as I am concerned, if someone posts a pic on the internet, for me its open season. They are perfectly at liberty not to download it and if they are so offended, they can just ask for it to be removed, I have no problem with that.