What stops that is the incumbents destroying or acquiring upstarts in their space. Etsy has already purchased Depop, Elo7, A Little Market, Trunkt, Reverb...
Why does Depop, A Little Market, Trunkt etc not get blamed for "selling out"? Because at the end of the day, people start businesses to make money. In fact, it is a tried and true model to create a start with the specific purpose of getting acquired.
