Twitter famously had a "fail whale" but it didn't stop the company from growing. If you have market demand (and I guess advertising) then you can get away with a sub-optimal product for a long time.
Agreed, but there's still an element of survivorship bias there. Plenty of companies failed as they couldn't keep up with their scaling requirements and pushed the "getting away with a sub-optimal product" for too long a time.
Friendster might fit though: https://highscalability.com/friendster-lost-lead-because-of-...
If it’s just “sign up any time you want and go”, yes, it can go that way.
If it’s “join that waiting list” or “book a call” (for KYC purposes or whatever), you have a buffer.
If user count is more or less constant (most internal websites, for example), it’s probably not an issue.
And so on.