Solas wants to destroy the world, likely killing a large quantity of people, if not absolutely everyone but any remaining dormant elves.
Like, that's not a good thing. No matter how you slant it. I don't think that's in question.
The 'I won't give up on you' option, however, does not agree with him - in fact, it involves challenging him on his bullshit and telling him to get a clue. Given that in dialogue with a friendly/romanced Inquisitor, Solas readily admits that his initial viewpoint - that modern thedosians aren't actually people - has been challenged by them, and that he freely admits that his actions are monstrous, and that he welcomes the Inquisitor challenging them, it seems that:
a) he's a confused mess, who is struggled to come to terms with how twisted his sense of morality has to be in order to achieve his goals, and really nowhere near as smart as he makes out.
his mind could probably be changed, given how bloody reluctant he is about the whole affair, and given that he actively encourages the Inquisitor to carry on with their work.
Thus, attempting to challenge Solas' ideas, rather than killing him/banishing him, is hardly impossible in the case of a friendly/romanced Inquisitor. The outcome: the world is not destroyed, everyone lives, is the same. The only difference is whether you feel Solas will receive justice, or will be forgiven. (Not touching that can of worms - it's not as important as saving the world).
Really, anyone who doesn't know Solas would probably just choose to stop him the easy way. Stopping him the hard way is really only a recognition of the good relationship the Inquisitor has with Solas, and a nod of respect to who he was as Solas, rather than Fen'Harel. It's not necessary. It's also not apologetic of his actions in the slightest.
DAI wasn't an entire narrative arc around Solas. Solas got a teaser-epilogue to reveal his importance and provide a 'gotcha' re-think of his character, and shared an epilogue DLC with two other major plots (the fate of the Inquisition, and Qunari).
Solas is introduced to give the player foreshadowing and insight of the future plotlines. The Inquisitor is no more required for this than Hawke was required to resolve Mage-Templar.
The Inquisitor has an established relationship with Solas, whether positive or negative, meaning that the plotline where his arc will be resolved would be more compelling dramatically if Solas was challenged in the end by the Inquisitor.
It's not required. It's just, in this particular case, it would likely be a more engaging ecnounter. Compare: the Inquisitor vs. Corypheus, a villain with similar goals, where Corypheus' character motives weren't developed, very little was established between Corypheus and the Inquisitor asides from 'grr, I'll stop you', and there was hardly any dramatic tension. One wonders whether it'd be better if Corypheus was replaced with a characterless force of nature - arguably, that'd be far more compelling. Solas would make a much better antagonist for the Inquisitor given the whole 'betrayed/lied to you by omission' aspect makes it personal, and just more interesting from a viewer's perspective.
my plan is to wipeout the dalish clan by clan till he gets mad....then ill start on the city pointy eared lot..then ill breed the halla with nugs..that'll make him think twice
Solas does not giving a flying **** about modern elves - excepting maybe the Inquisitor, if they are one - so this would be pointless.