Now that I've actually seen the ending- i.e., Trespasser's- I can see how people could contrive a DA4 plot involving the Inq as protagonist. I mean, with the imagination you can approach it any way you like, but there is a way to contrive a story arc upon the Inq's resolve about Solas despite that the Inquisition has been neutered (become a wing of the Chantry/ dismantled)... and despite that the Inq is now missing an arm... and would be starting at "should-be-27th-lvl..." After all, it's only the Inq (and close cohorts) that even know about Solas. (Even the qunari had no clue he wasn't just an Inquisition agent.)
But I can also imagine a story where a new protagonist meets the Inq, is told about Solas and asked/recruited/compelled to infiltrate his organization, and then throughout the course of DA4 the new protagonist gets to know Solas in a different way than the Inq ever did, gets close enough to him to argue with him and/or come to agree with him. (Not that the plot has to be about infiltrating Solas' organization, but still...) That would make the end choices regarding "redemption/stopping" no less gripping- in fact, potentially a lot more fleshed out since the main plot would involve the protagonist's Solas-confrontation centrally and involve a narrative process of learning and growing into the part required. And then it's the new protagonist- with all the new info and how the player experienced Solas when he's not deceiving anyone and his story is understood- that has to decide how to handle it- persuade him or assassinate him... or just foil his plans somehow, whatever those plans actually are and whatever the foiling would entail (since we don't actually know). A cool plot twist might be that Solas saves the new protagonist's life a couple times along the way, making you roleplay an interesting relationship with him that deals heavily with the theme of betrayal. Or a twist that at the end, after having risen through the ranks of Solas' organization, he tells you he was aware your protagonist was an Inq agent and gives the final choice about whatever his plans are to you rather than needing to be thwarted: "It is your world. You have seen what is at stake. I have helped you to understand what the consequences are, what will be lost however you decide. It is not for me to decide..." *vanishes* In any case, in that scenario it could be completely satisfying to be a new protagonist who nevertheless determines the fate of Solas' narrative. Sounds a lot less simplistic that way, a lot more character development moments to experience than being an Inq who's not much better than a Spirit of Stop/Redeem Solas with no more depth of character than single-minded Justice.
I suppose I primarily reject a returning Inq as protagonist just because I dislike the "instant hero" identity- requiring very little depth of personal involvement for the player. After having endured it throughout DAI- and with a character that lacked any substantive background as it was- that same hamfisted identity would simply be extended further into DA4 which just belabors an already tired storyline. As I said, it could be done, but it just seems a lot less interesting (among other things) than just doing what DA has been good at: introducing new protagonists with an engaging story of their own who find themselves thrust into events larger than themselves. New players who never played DAI, of course, would benefit from getting to know Solas differently than the Inq did rather than having the DAI experiences grafted into one's DA4 identity and presumed. And returning players with a new protagonist would be able to get the whole DA-immersion experience without having everything be about "Me 'n' Fen cuz we's sooo important."