Your post makes sense, but, if there are over 1000 other potential providers out there, how many would have features, capacity, etc., that would make sense for (or even be workable for) Parler?
How many could / would stand up to being pressured not to do business with Parler?
How long would it take for Parler to make the switch?
One thing is for sure, Parler remains offline @ present.
GAB appears to be online but frozen or totally overwhelmed.
Capacity? All of them. These providers could each host 100,000 Parlers. Features? Some of that depends on Parler's staffing. For example, if their people only understand how to run a service like RDS and don't - for example - have any experience managing hypervisors, they're going to need to stick with providers that have those services available. But that's a simple staffing thing, and they still have about a dozen different providers that will have those types of services readily available. Or they can hire better IT people who have broader knowledge and use any of the other 1,000. There's nothing AWS has you can't easily do yourself or do on Microsoft Azure or Google Cloud Platform or any other.
"How many could / would stand up to being pressured not to do business with Parler?"
But that isn't relevant to a court case alleging that Amazon AWS is a "trust" or a monopoly. If the thing you're wanting to host is so hot that nobody wants to take your money, that isn't an issue of monopoly. The trust issue comes in when you're basically the only game in town to the point that you can pick winners and losers. That doesn't exist in the hosted services world. AWS is big, but less than half of the cloud platform market and probably less than 10% of the overall hosted services market.
"How long would it take for Parler to make the switch?"
Again, that's not relevant to a case alleging anti-trust activity. If my landlord kicks me out for keeping pets in violation of the lease, I can't allege anti-trust activity because it's going to take me a while to find a new place and move all my stuff. Well, I can allege anything I want, but that's getting thrown out unless I live in an area where there's only one landlord and he's refusing to rent to me. But there isn't. There's 1,000 landlords. How long it takes to move my stuff between them is not an issue relating to monopoly.
"GAB appears to be online but frozen or totally overwhelmed."
GAB is likely experiencing either scaling problems (platform grew quickly and they didn't build the platform in a way that can grow quickly) or budget problems (adding more capacity costs more money - possibly more money than you have available) or both. Still not relevant to the legal case with Parler. Parler has no legal case. What they have is a lawyer willing to take their money.