Well, it wasn't "clear". But fine, sorry, i misunderstood you.
No one has said DA4 should definitely have the Inquisitor (and if they have i would disagree). What MOST are saying is Solas's story is the Inquisitor's story. We feel that it would be for the best if that story ends with those two characters. Solas tells you that you have only a "few" years left. An age in Thedas is about 100 year, so skipping lots of time isn't really an option since it's about a "time" and "place".
Geez, everything i'm saying i'm saying from a story perspective. I'm trying to write a book (i'm a fledgling writer, sure, but the methods are the same) and in my story there are 2 people who could stop the antagonist. The protagonist would be the most satisfying ending due to the EVENTS, but i could decide "Eh, let him do it" instead. It can be done, but it will make my plot so weak it isn't worth it.
Solas and the Inquisitor started their story together, they are each the most significant character in the Inquisitor's arc. They can bring in a new PC at the cost of the character motivation, if they wish. Hell, they may shock me and do it magnificently. But they should not feel obliged to keep or discard a protag because they have said it will be X way.
And for goodness sake, if it is about Thedas then we should just be a tree and watch people do stuff, since it's about the "world" and not the "characters". That is such a weak argument.
The problem is that Solas has been around longer than even the Blights. Yep, he messed up, and joined the Inquisition to "fix" it, but the reality is, the Inquisitor and the Inquisition are the equivalent of a footnote in his history, even with a romance.
So let's look at this from a writer's perspective, because a player's perspective is limited to what we've been presented:
In the beginning there were the Elves and Spirits. These beings co-existed, note here that we even have a reference to this in Solas dialog before we're ever driven out of Haven, only in the "What if x" scenario. There was a war, not clear on the details, and I'm not sure many were provided, but there was one. The end result being that some of the prominent leaders took on the mantles of Gods. To prevent these false gods from hurting the people more, Solas hurt them worse, by separating them from the Fade, and cutting off their magic/immortality. This is the beginning, not "Hey look, someone fell out of the Fade". How many thousands of years did Solas sleep? I didn't get any clear reference points to judge.
This is the story that's being told, or will be told, going forward. We got some of the ground work, but the Inquisitor's ancestors weren't even born yet, let alone the Inquisitor. This isn't the Inquisitor's story, just their arc in Thedas' story. Just how important is the Inquisitor to Solas? "I wouldn't sleep with you under false pretenses", he actually says that to a romanced Inquisitor, "but I'll kill you in a heartbeat to restore my people". Hmm, doesn't seem like his priorities are in the right place to me. Solas is immortal, the PC isn't. Her entire life isn't even a drop in the bucket that is his life, and he wants that back more than he wants the Inquisitor, he made that pretty clear when he did whatever it was he did to her arm, and then walked away. The obsession with both past PCs and LIs not withstanding, nothing in his actions states that she's more important to him than restoring his people, even though he knows it's going to cause the world to burn. He even lays this out in dialog.