I see no issue with using temporary sets for special programmes - if anything, I think it would less imaginative to use the same studio for absolutely everything.
The Glass Box isn't particularly versatile, but it could be if the rotated the desk once again and made more use of the Sky Centre backdrop.
Westminster actually is quite flexible, all they need to do is wheel out the desk, swap it for soft seats, or a different desk etc. The screens can display whatever is needed.
I can understand Sky News's switch to a more white/clean studio style, if to try to convey more of a focus on the content rather than a gaudy studio, which to be fair, the Sky News Centre did steer towards. Still, one can have a clean, content-focused presentation while still conveying a more authoritative identity - and indeed an identity full stop.
I just think a decision needs to be made on whether Sky News is a rolling news channel - in which case, there's no point in having different titled programmes - especially as they are all currently identical in terms of news stories and how they are presented (save for one or two programmes having a standing presenter).
Or, they need to give more editorial independence to some of the programmes, with different graphics for each. And yes, that does sound like appointment to view, but I would much rather a proper ATV rather than the slightly fake style at the moment. Breakfast, Today, Afternoon, News Hour, Tonight, News at Nine and so on are all virtually identical. Just a different face reading the news and mildly varied title cards.
Programmes would feel more special if they focused deeper on a different story or something. One hour programmes (but less of them) that are actually unique would be more appealing - in my opinion at least.
I actually really liked the podcast special with Dermot that they broadcast on screen, something like that would be great.