I agree with you, I was just using them as an example since they were mentioned prior and because "one-shots" and more contemporary picks are such a point of contention these days. Sheik ought to be judged differently as an extension of Zelda for a good decade too, but I feel like if they were added today the kneejerk reaction would be to treat them as expendable to be cut loose next time. Despite how fresh and original their playstyle is, the interesting dynamics they bring to the roster and so forth.
Hell, a lot of people are already treating Sheik this way even with the legacy. Not saying they're 100% safe but I think your latter point rings true and is a great reason for Sheik to remain a part of the series. 64 and Melee characters do have a slight advantage in the way they've founded character archetypes and playstyles that would be built upon and iterated throughout... so from exclusively that angle Sheik feels about as expendable as Fox or Marth, which is to say not very.
This all just stems from the notion that characters are here to "represent" games. To most Smash speculators, SSB!Sheik's loose connection to OoT isn't enough to carry him, because there's a better and objectively more important OoT representative in Ganondorf. However, once you realize that most of these characters are here to represent themselves, it becomes a lot clearer that Sheik has merits both in his series in in Smash.
Not saying that game representation doesn't matter. Rosalina is a good example: she was picked exclusively as a Mario Galaxy rep, as mentioned in the SSB4 project plan. However, she has enough of her own identity sprinkled in her moveset that she, as the closest example of a "game rep",
still feels detached from the fans' notion.
Tying this back to the ongoing convo about Lyn: she can both represent herself and FE in a really cool way. Adding to the fact that she complements Marth & Ike's Smash movesets and designs, she also rounds out the classic FE cast in a really nice way. Marth, from the first game; Lyn, from the first international game; Ike, from the first home console/3D game. That one is a bit arbitrary, but adding to the fact that she's a consistently popular character, I'd say she has enough reasons.
I can see where you're coming from, but I'm just simply one to go with information we have available to us and precedence to base my predictions around. Can precedence be broken? Of course. But it would have to be a really unique occurrence that frankly, I can't see in the case with Lyn, especially when she's not the only one the argument could be made for (i.e. Edelgard).
Edegard isn't really comparable to Lyn. Byleth specifically got in as a Three Houses representative, so Edelgard would be redundant in that regard. Also, besides Awakening's special circumstances (AKA echo fighter luck), there's not a single FE game that SSB double dips in. Not to mention that SSB Byleth already uses Edelgard's weapon. Lol
Lyn is from an unrepresented game that can also be counted as sort of a landmark for FE (first international release). She's been consistently popular for the past 15+ years, whereas Edelgard's hype has worn off, especially after the game came out and it was revealed that she... you know. She's contentious and, uh, morally gray, which is interesting. But still doesn't compare to Lyn's prevalence and popularity.