You do realize it can become indefinite right? It is 90 days only if the countries share 'data' which the trump regime poorly defined. And its highly unlikely countries like Iran would accept that.
It'll start with this 90 day Muslim ban and devolve into something much longer and much worse.
Legally, it could be, but politically, it probably won't.
This temporary ban is political theatre.
First - migrants from different parts of the world already face very different degrees of scrutiny - this has been ongoing for a very long time, certainly under Obama etc.. - so the issue of 'national discrimination' will hold up under legal pressure, I think.
Second - the commenters point is pragmatically valid because this exec order is a political means for Trump to show that 'he's doing what he said he would do' and to give impetus to his base. That's mostly it. It's not a real operational measure.
After 90 days, Trump will declare that 'we've made changes to the vetting process and have 'extreme vetting' (even though there was already 'extreme vetting' under Obama) - and this will be over.
Caveat - your point about Iran is valid - but Iran is a very different country than the others, and is on the list for different reasons.
I believe that stronger restrictions may be put in place for Iranian citizens after 90 days - this is the beginning of the 're-establishment' of a new Iran-USA relationship which will go in the opposite direction Obama had it going in, for better or worse. Probably worse, but hey. Remember that Trump promised to 'end' the deal that Obama made with Iran - and this ban is going to be part of Trump's 'negotiating process', if you can call it that.
It may be worthwhile to legally challenge the order anyhow - for local political reasons (institutions and governments want to seem like they are 'doing something about this injustice) - but also to make sure that, as you say, this could go on for longer than 90 days - but I doubt that.
Anyhow - it's important to look at this from a political perspective as much as a legal one, because the game that's being played is mostly political.
You could be right, but I'll bet $1000 that almost all of this is going to be legally moot in 90 days.
Listen, I don't like Trump, but I think we have to sit outside the political game to see it clearly.