It's interesting that you're using slither.io as an example. I've played hundreds of io games. Almost all of them are trash but figuring out the mechanics and finding the surefire way to win and then moving on to the next game was what I found interesting. So far there is only one good .io game that I actually keep coming back to and that is starblast.io.

My takeaway is: If your game fails then it simply wasn't good enough and making good games is hard.