Nailed it with first sentence: it takes 10k users to run an app with 10k users! And that will be, by far, the hardest part.
The technical stuff will be pretty much trivial. Any decently constructed app on any decent framework (Rails, etc) on any decent host (AWS, DO) would be able to handle a 10k user app (probably maxing out at 1% online at same time) without breaking a sweat.
And you will have plenty of time to build out the tech because it will probably take you many months to get to even a few thousand users (depending on what kind of app it is, of course).
The technical stuff will be pretty much trivial. Any decently constructed app on any decent framework (Rails, etc) on any decent host (AWS, DO) would be able to handle a 10k user app (probably maxing out at 1% online at same time) without breaking a sweat.
And you will have plenty of time to build out the tech because it will probably take you many months to get to even a few thousand users (depending on what kind of app it is, of course).