I really enjoyed reading this. Much like Instagram, which had thousands of users sign up on the first day, if you aren't able to scale because of your skill level, wouldn't that affect usage and lead to comments like: 'The app/site is so slow'?
It depends on the adoption model.
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.
Aren't comments like "the site is too slow" similar to "the city is too crowded"?
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.