That depends on how different the Echo is to the original. Chrom, Lucina, and especially Ken all differ enough from their original counterparts to need more fine tuned balancing. It may be easier to balance them overall than a full character, but there's still plenty of work that goes into making them work properly. Not to mention, almost all of the ideas surrounding Echoes seems to be more on the Chrom side of things as opposed to the Richter side. People want characters to be more unique.
Once again, bringing up the excitement of the Mii costume is a horrible false equivalency given his specific set of circumstances that literally no Echo will even come close to. The success of Sans is specific to that character, much as the success of other additions to Smash would be because of the character that people are interested in and very few of the Echoes even provide the opportunity for recognition, let alone hype. I don't think people will be outraged (I mean, some will because this is still Smash and I do think putting development resources towards Echoes is a more legitimate thing to take issue with), but I do think on the whole people will be more apathetic about their inclusion and they are never going to be able to even achieve a portion of the hype that is critical to post game support that full newcomers will.
I mean, the options you're giving for Echoes reek of padding the roster with mid-effort clones. Kasumi has yet to be proven as a character on her own merit/people are probably not going to be super excited about another Persona 5 character. Street Fighter is already the third party series with arguably the most representation in the game, so I'm sure just padding out the Shoto clones even more will not go over well. 4 Marths? How about 4 Ryus? (and Evil Ryu would be as bad if not worse than Dark Pit I feel like). Zack Fair isn't exactly some beloved icon in his own right either as mostly just an extended FF7 lore type character.
All these characters just serve the purpose of appealing to existing niches and don't really have extended excitement factor. They lose their value substantially when presented on their own without a larger base roster to back them up. They only kind of work if you present with another character... but at that point, why are you throwing additional resources behind a character you know is just kind an addendum as opposed to its own marquee thing? I just don't see Nintendo jumping on the idea of Echoes. Yes, they are inherently cheaper to produce, but they are less profitable (you would have to sell them for less than a full fighter AND they're less attractive to the consumer as glorified alternate costumes), fundamentally cannot achieve any expansion into new universes, more or less have to be B-tier and below characters in terms of popularity and recognition, and don't satisfy much fan demand. I just don't see any reason for them to really bring enough benefit to take any amount of resources away from full characters at this point. They were an elegant solution to a problem exclusive to the base roster, now they would function as inelegant padding given that time, money, and priority are not the problems they were for base game.