You haven't tried D3 yet vs Marth?
I did, I even went through the bother of getting the chain grab down LOL. The problem with both D3 and ICs in the MU for me is that my playstyle is generally... well more extreme. Either I am camping like a ******, or I am rushdown- I have problems really playing the middle ground, especially vs a character that can combat my spacing.
If MK hard camps marth, they will just wait and **** you. If MK hard rushes marth he will just get grabbed. I still think MK wins that MU solidly it's just past a certain extent at high level I don't feel myself growing at the MU anymore. I could just copy how someone else does it I guess... but its just not in my general playstyle/mindset.
My diddy already has most of the tech skill, and knows what to do in most situations from playing ADHD it just needs to get in the experience // fluidity. For me, putting that kind of work in so that I can enjoy the game seems much more enjoyable than frustrating myself w/ MK atm...
I will still continue to practice and learn the MU, and one day maybe it will click but atm I'm not getting better at it so it's time to try something different lol.