> But how will they reach critical mass?

Tildes prioritizes quality over quantity.

> How to avoid being popular driving it into trash like most other aggregators?

It's currently invite-only to limit its growth and keep its userbase from being polluted. Later, a trust system will be added for active and trusted users to help moderate it. When it gets opened to the public, it might also limit signups to prevent overwhelming floods of users taking over.

Also see https://docs.tildes.net/overall-goals.