An astute observation. Yet people do not assume that HN will rival Facebook, nor do they place valuations in the hundreds of billions on it.

Twitters troubles come from a misguided impression of what it is supposed to be, creating pressure on the product to not be what it in fact is.

Isn't this a common concern with most startups which seek to grow beyond 6, 7 or 8 figures?

It is, but just like your local neighbourhood cafe knows they will not be an 8-figure company, so should a lot of tech companies.