If you, on the other hand, want to write good, performant, maintenable and (relatively) bug-free software, then there are better choices.
You can do that part when the money comes in.
Once you invite the investors in, the company isn't truly yours, and they expect 1000x growth. There will always be higher priorities than a rewrite.
If you don't take investors' money, then it's theoretically feasible, but, in practice I've seen companies stuck with their Python/Ruby stack for 10 years or longer, always wanting to rewrite but never having the time. Microservice architecture helps (not that it's not a can of worms of its own), and these companies tend to write new functionalities in their desired language (such as Go or Scala) and deploy them as separate microservice. The old code remains in Python or Ruby though, and now they have a multilingual mess that is near-to-impossible to maintain for a small team. Why not start with a better language in the first place?
Dev teams that are unable or willing to properly architect and test their projects will end up with buggy and difficult to maintainable code regardless of language.
> Most startups that YC has funded that became successful (Series B or higher) were written in Python or Ruby.
It depends on what your goal is. If you want to get rich off of VC money, Python and Ruby might be a good fit. If you, on the other hand, want to write good, performant, maintenable and (relatively) bug-free software, then there are better choices.