I agree with you somewhat. I don't agree with the MU system we have now (but for different reasons), and there is no "100% impossible MU" in Smash, but there ARE 'practically impossible MUs' at high-level. Examples are Fox: Pichu (Fox out-camps and out-spaces Pichu easily, but the MU breaking feature is that Pichu's techroll goes nowhere. If Pichu is knocked down by anything, such as dthrow or shine, Fox can techchase him with dthrow until he hits death %'s and dies at 70% from usmash), Sheik:Bowser in Melee (Bowser WILL shield a lot but Sheik has one of the best grab games in Melee, and Sheik can CG him from 0-to-death, ending the CG with a fair), and DDD:Bowser (Bowser can not reasonably avoid being grabbed at all, not if he wants to actually land damage on DDD, who outspaces him, anyway. After being grabbed, Bowser is small-stepped CG'd to the ledge, where he is ledge infinited. At death %'s, dthrow can be combo'd into dtilt, bair, usmash, and dsmash on-stage so DDD always has a fresh killing option, even though they should all be fresh after the infinite anyway...

). There
are 'practically impossible MUs' and +4's should be conserved ONLY for them. As it is now, our MU chart actually has too many untrue +4's.
I actually don't agree with the +4 on MK:Ganon. That MU isn't 'impossible' like ICs:Ganon is. Heck, I'd even be willing to say that DDD:Ganon is harder for Ganon than MK:Ganon. DDD actually has the choice to automatically erase one of Ganon's stocks at ANY % from a correct read in a 50-50 situation after a CG > dthrow at the ledge. DDD also doesn't die nearly as early from hard 'Ganon-reads'.
There is no 100:0 MU in any fighting game. At worst, I'd say the -4 MUs in Smash range from 90:10 to 95:5.
-3's are "hideously bad". You have almost no chance of winning those matches against a player of equal skill in tourney. -4's are different because in those MUs, you have almost no chance of winning against ANY competent player who is aware of the MU's MU-breaking aspects. Btw, I don't think you're
allowed to have your own "perception" of what each MU value means. There is
supposed to be a "clear answer" to what each MU value means. We have an official scale and if we can't completely agree on how it's interpreted, then we can't discuss MU values at all and the chart promptly holds no value. In the MU panel discussions, we must agree to a 'common viewpoint' before we start firing off what we think the value should be. Where we disagree is on how hard/easy a certain feature makes a MU, not what the values are in the first place.
Ftr, THIS is our current MU value system:
Notice how a -3 = large disadvantage/
hard countered. Do you know what "hard countered" means? That's "hideously bad" because it means that the countering player has chosen a char that is meant to shut down any advantage the countered player may have. Anything else is a soft counter a.k.a. a character that gives you a (clear) advantage but still requires you to outplay your opponent, given stage choice, etc.
I don't think there is really any other way for DDDs to play that MU.....