So this is basically a "Kill X character because I hate them" thing? It's gems like these why I'm the happiest when game studios avoid doing fan service as much as they can.
Granted, I'm a huge Morri-fan. But even so, I just can't comprehend the active, dedicated hate towards a companion character that fuels reactions like this one. Sure, I disliked plenty of DA characters before (Wynne, Valenna, the entire DA2 cast, even Solas until the ending etc.), but I would never want to kill any of them just for the heck of it or throw a hissy fit on forums because I can't do so. Every single one of these can have great story potential if the writers will it, obnoxious or not. I may have wanted to punch Fenris in the face 90% of the time, I still wouldn't mind seeing him return as an NPC with some new role (or yes, even Anders for that matter). Quite the contrary, they might even positively surprise me like they did with Leliana for example (I disliked her 'prayers and holy water' self in DAO, even killed her in one playthrough, but adored her in Inquisition, even dreading that she wasn't a companion this time around). I've seen people having this same experience with Morrigan, actually (they disliked her in DAO but warmed up to her in Inquisition; she was definitely more humble towards the player character even in a not-a-mom state). Solas was actually annoying me to death during my first playthrough, but I gained a newfound appreciation for him after the big reveal and can absolutely not wait to have him as the big bad next time, killing option or no.
Having Morrigan return didn't tie BioWare's hands in introducing new plot-relevant guys like Solas. Having Leliana and Cullen in the War Council didn't "remove space" for new additions like Josephine. Having Varric in the party didn't take the spotlight from the rest of the bunch. Where are the "Kill'em with fire" threads for those ones, I wonder?
In the same manner, Solas and Morrigan reappearing-without-dying will not somehow siphon out the creativity from the franchise, Thedas is quite a bit larger than that, thankfully. Some characters are just written to be more relevant than others, that's how fiction works, and I'm completely fine with that. That doesn't necessarily make them "pet characters", which pretty much equals "relevant characters I don't like" around here.
With that said, story and character fatigue is an issue Weekes and co should definitely be considering. I do in fact agree that existing characters should have a 3-game life cycle in most cases (possibly with a few larger-than-life exceptions like Flemythal) and I wouldn't mind if the next game would be Morrigan's last appearance, or DA5 in Solas' case. I have nothing against them dying or optionally dying if it's done in a tasteful enough way (commiting suicide Saren-style, either of them making a noble sacrifice even, it's not beyond any of them), but not as a bone thrown to haters. If they're written out to live as hermits/go wander around the world/settle down for good, that's fine too.
Unrelated request: I'd die to have Kieran as an NPC next game. Potentially-dead people did have roles in previous games, it shouldn't be difficult to do so with a potentially-not-existing one.