They went out of their way to explain to us why the Inquisitor wouldn't be back: While the first step isn't really all that limiting, they did take their arm, and then there's dialog at the end where they state they can't be the ones to go after him, because he knows how they work, and they'll have to find other people. So yes, they'd have to twist the story they told us out of proportion to bring the Inquisitor back. It would require functional amnesia on Solas' part, since he'd have to forget how you operate, despite being with you all the way through the main game of Inquisition, otherwise, his spies would have all the information he'd need to beat you right after the prologue.
On the contrary - they went out of their way to suggest that Inquisitor is not yet done, by serving us a delicious narrative twist.
Think about it: we've spent a good deal of Trespasser being told that Inquisition may not be needed anymore, that perhaps it's time to put the sword away and move on, and worry that maybe Inquisitor is about to die from malfunctioning Anchor....only at the very end for Inquisitor to survive and being given a new purpose with deep personal stakes.
Many people have also already pointed out that removing the arm with the Anchor is also a good reason to knock Inquisitor back to square one - they don't have a unique power that makes them OP anymore and they have a good excuse to put them back on a lower level, so they can re-learn how to fight.
Also... I don't know, it's like you're willfully ignoring what was said in the very game itself. Yes, Leliana said that Solas knows how Inquisition in its old shape has worked. Solution to it? Well, we have to find new people to work with!
Not only Inquisitor uses the word we (will have to do this - same with Cassandra: we will have to be careful), not only it's he/she who points a direction to achieve this new goal, but the message through epilogue cards and many dialogue options in the end suggest that Inquisition and Inquisitor have gone the route of a secretive order, upholding a lie of the organization being diminished or retired, while its leader travels all over the world, working on something.
You know, I'm really struggling with the mental gymnastics required to go from "We can't be the ones to fight him, he knows how we operate" to "We can take him, just put us in the next game".
I don't think we'll reach the agreement as to who is doing mental gymnastics here.
I mean: give me an example of any Thedosian organization that has any inkling what Solas is up to, know him personally or have any shade of chance to guess what he can do?
You think that Solas only has his agents in Inquisition? Judging from events in Trespasser AND Solas' earlier meddling with Orlesian leaders (in The Masked Empire) we can safely assume that everyone is compromised, while no one else has absolutely any idea who they're dealing with.
Just look what happened at the Exalted Council - who has dealt with the Qunari threat, while everyone else was either blissfully unaware or put their heads in the sand? Inquisition might have faltered same as others in terms of not screening their agents and getting infiltrated, but they've effectively saved the South. Again.
Therefore while Solas may have played everyone for fools, the truth is Inquisition knows him best as well - it's a double-edged sword in that regard. So far it seems no one else knows who they're dealing with, much less where to start doing anything to stop them.
I mean... you think Tevinter would? When the game makes it really clear that they'd be preoccupied with the Qunari invasion as well as a struggle for power in its own government?
Who else is there? The Wardens? When it was established already that there's a terrible divide in the Order and that they might be at the at the brink of destruction?
The Chantry? Orlais? Ferelden? Both nations did nothing but made Inquisitor's life tedious and did nothing to help with Qunari infiltration - that much we know already. And what they could do against Solas? Nothing really.
And Leliana and Cassandra seem to be working together with Inquisitor in secret, while officially they have to deal with re-building if the South and Chantry itself.
Really, unless we're being introduced to other forces at play, so far only what's left of Inquisition seem to have any realistic chance to uncover what Solas is planning and have any chance to stop him.