Maybe I should count myself lucky to have never used either of these websites. Never even heard of Mangarock until it was taken down and brought up here, and never touched KissAnime/Manga because: for anime, I've always used fansubs if not available where I lived; for manga, when not reading official stuff, I used Onemanga (remember THAT website before most of it's well-known content was taken down?), then moved on to Bato.to (before it was effectively taken down/sold?), and then moved here. I hated most manga aggregators and would avoid most, unless I was looking for something VERY particular that no one else seemed to host, and I hated streaming anime because why do that when I can store it and become a literal archivist. So far, with the few things I've managed to save, there is a LOT of really old mid-late 2000s era fansubs that can't even be found anymore (dead links/torrents/websites/fansub groups/etc.) of stuff I nearly forgotten lol
But yeah, sounds like things will get pretty wild for a while. Tempted to host stuff for Mangadex@home, but I have to clean one of my 4TB HDDs and see if it's healthy enough to even do so. Not to mention, need to build a NAS soon with some Red/Enterprise drives, so who knows.