The challenge is growth. Employees of so called "practical" startups, will compare compensation/wlb options of working at the startup vs fast growing unicorn. Without growth, it is hard to keep great talent (at least that has been my experience).

Also, big tech/unicorn can just spin up a new product line to compete in the same market as the startup. In some situations, they might sell for loss to gain market share (e.g. microsoft teams vs slack).