So, I mean, good luck.
If you can't make that combination work well, there's another place to look for problems besides your tool kit. You might need to ask yourself if you really understand the tools you're trying to use.
But like I said, this has always been a very cool project. My "good luck" was meant more as actual good luck than a Morgan Freeman You're-trying-to-blackmail-batman kind of good luck.
pypi.org is another that's familiar. You know, every time you type `pip install x` yeah, that's pypi.
Although I think those are both powered mainly by Pyramid rather than flask. Still, same concept.
As others mention, large parts of google and youtube are still python. Dropbox was so invested in python that they employed Guido van Rossum for a while. Instagram, a lot of Yahoo! back when they were a thing, Spotify, Quora, Pinterest, Hipmunk, Disqus, and this really obscure satire site called The Onion that totally never gets any traffic at all.
All of them powered by python at their core, many of them Django, some Pyramid, and some Flask.
Yes, getting that big does require big teams. Becoming one of the top 100 or so sites on the internet always requires some special sauce as well as dedicated teams. But most of these companies started with Python and a framework and got to massive web scale along the way and never changed the core platform because there really wasn't a need. Handling scale isn't about your core language or framework. It's about dozens of other things that you can offload to other things if you're smart. But let's be real: sourcehut isn't close to any of that level of traffic.
My negativity on this isn't about stanning a particular language. I'm an agnostic in multiple ways. I'll use whatever tool seems like the best fit. I'm down on this because the explanation is tool-blaming, murky, unclear, and doesn't provide a lot of the detail I would want to have if I were depending on this service.
On the other hand, the guy has always said this is an alpha project and you should expect major changes. That's all fine. It's just weird to me to see a "why I'm changing from X to Y" post that doesn't really explain anything other than "I might be bad at this."