There is no evidence that they did so, so this argument that they can be replaced doesn't work.
Yeah, see in my playthroughs there are no such empty towers, because I'm more afraid of the mages you apparently see no reason to fear despite them not even having to decide to go on a killing spree. I can see an argument for confining Wardens I guess, but how do you see an argument for confining Wardens and not mages?
One thing I will point out in the Wardens defense is that the bindings would have stopped the demons from threatening to extinguish all life from all we can see, unless that's how they were used. Yeah, using demons is really stupid and desperate, but I will point out that given what they thought they were being threatened with, this plan makes sense assuming that the bindings are secure.
The Wardens think that everyone who knows the secret of why they are needed is about to die. They don't know why it's happening and can't be sure that people who take the Joining later (after they all die) are safe. They can't be sure that any warrior can be empowered to kill the next Archdemon and avert the apocalypse that's due in a few centuries. If the situation they thought they were reacting to was real (and the binding ritual had no strings attached) this would be an acceptable response to it.
The possibility of replacing the Grey Wardens is an okay head cannon theory (not argument) for some play-throughs RP later world states. I don't remember anything in DAI that would indicate any likelihood of it happening. It just makes sense for some as an idea/reaction after playing DAI.
IMHO it would be relevant DAI game world info to address in-game if the Grey Spawn joining ritual specifics were found after defeating the Grey Spawn at Adamant fortress. If you banish the Grey Spawn even if it is temporary it seems relevant to have at least considered a Plan B if the next blight happens. It a biggish game choice to make without info you would reasonably expect. I'm still happy they allowed us at least a limited choice of what do with them.
Honestly, I've completed DAI twice and first I allied with the post demon army Derp Wardens and the second time I banished them. I really liked that in-game we could call them on being stupid and we could make an Inquisition limited choice in dealing with them. However, Grey Wardens being stupid aren't like Orsino being stupid at the end of DA2 since Thedas thankfully doesn't need an Orsino to save them from the next blight. I wish DAI dealt more indepthly throughout the game with the Grey Warden issue since they are crucially important to Thedas and the DA storyline. Only having a knee jerk limited exile option oddly ignores a lot of blighty issues that seem relevant. It will be interesting to see what Bioware does with the wardens in the future.
I was more joking about locking the Grey Spawn in a circle tower since there was now unused space. If the option to lock them up for a period of time had even been available in-game it would have of course been way more entertaining to lock them in their pal Cory's old prison.
I haven't decided yet on my acceptable world state if DA4 is made and I continue the DA story. I'll probably go with a DAI where I got pimplar support first so I guess my post DAI circles and mage academy will have a lot of rebel mages turned evil venatori cultists since it seemed like the rebs were the only mages left in Thedas when I needed a few to close that sky hole thing. It didn't go well in DAI when evil Venatori palled around with the Grey Spawn so probably shouldn't lock them both up in the same tower.
As for also locking up the mages if you lock up the Grey Spawn that probably would work as badly as it did for the mages so far.
My opinion, is the Grey Spawn as presented in DAI didn't work out solely in the best interest of the DA world. I think locking them up and using the Inquisition Comfy Chair until they give up their joining secrets is funny to talk about but unlikely to happen until years from now when they release the DAI enhanced edition.
There are a lot of story loose ends like what will happened with the Grey Spawn in DAI that could make a future game interesting. I would only be disappointed if it wasn't addressed in an interesting way in a future game but Bioware has a lot story loose ends to address if there is a DA4 or DA5. Orlais or another country creating their own tainter order might be interesting if done well and relevant in a future game.