I like platformers, but they are far from my favorite game type, so I will have some bias.
Judging from my experience playing until world 2, the comments of the other playtesters (where one made it to world 4), and comparing those with your detailed posts about progress on worlds 5-8, I think it's worth considering something.
You may be developing game that is more fun for you to develop, than it is for others to play.
I'm sure you are learning a lot about game design and development during this process, and I don't want to say anything to discourage that. But I do think you should hear from someone that - the content you are working on now, we will only see as screenshots you post. Your volunteer playtesters haven't reached it - most or all never will. And some of us really try, even when the experience stops being fun.
Something to keep in mind is that people aren't going to play through content just because you made it. You actually have to listen to their feedback. When you invite playtesters to your game, and they say that the difficulty curve is too steep in world 2, go fix world 2. If you instead focus work on worlds 5-8, you are not fulfilling your end of the bargain. That's a fast way to run out of playtesters.
Anyway, keep at it as long as you find it fulfilling, and good luck with your future games.