In theory having easy more organic multiclassing ought to bring alot of adaptability back it makes sense and maybe MCing takes more game fu with fifth edition to use well than I have.
3e-style MCing is an elegant solution to the thorny problem of class straightjacketing, but to really work well, each level of each class must be balanced with every other level of every other class. That is, if you treat class-levels as building blocks, you better make 'em cubes. Which, when D&D has rarely managed to even roughly balance any two classes in any one edition, is a really, really tall order. Bottom line: 3e style MCing works great, but the only class design it actually works great /with/ is the 3e fighter.
MCing in 4e when two very odd ways. There were the MCing feats, which essentially charged you feats for the privilege of learning powers that probably didn't synergize too well with your class features, and Hybriding, which, just, well, it wasn't as bad as Gestalts - I suppose it was a fairly balanced/playable take on classic D&D MCing. 4e probably could've handled something a lot closer to feat based MCing, just without so many feat taxes to do it. After the initial MC feat, just retrain powers in from that class with some simple stricture, like only one each of your EDU powers, and never your highest level power of that sort, and it uses your retraining for that level.
Would 4e have gone there given a chance to continue developing? I can't imagine why, unless there was some sort of dramatic Feat Purge Udpate to winnow away the chaff... and that'd've been a /change/ in direction.