I don’t know about Japan, but generally laws do not require the copyright owner to respond to violations.
But, yes, I would not like it if people took my work. Too often I’ve seen true art by hard-working people be stolen and reposted on Instagram. Said reposts then end up receiving more attention than the original.
Or the cases where these remixes ruin the reputation of the original franchise. That’s why Nintendo has such an attitude. It’s because most fan-games are complete shit.
If you have the strength to make a game in the first place, you should have the strength to either get more workers or work on the art, music, story and programming yourself, instead of copping out and copying.