Sure, but in practice if you have any complexity in your stack then it's going to be a PITA to from Azure to AWS (or vice-versa).

If you're just running instances then maybe it's not too much work - though you're still going to have to think about backup / restore, access control, autoscaling (maybe), cost tracking / allocation, etc...

I'm sure Microsoft are banking on a profitable percentage of YC companies becoming so wedded to that Azure systems that the opportunity cost of switching to AWS is too high.

Edit: Typo