In actuality, how many startups have switched from <cloud> to an alternative setup? Truth of the matter is that once a startup gets to a certain size, they use these to lock in savings versus moving to bespoke infrastructure: https://www.parkmycloud.com/blog/aws-edp/
A lot of startups perpetually run on VC money and don't particularly care about becoming profitable. But those that do make it sometimes shift at least some of their infrastructure - see Dropbox or Netflix (with edge caches), Cloudflare, etc.
It probably depends a lot on whether the startup is selling a service or collecting eyeballs to sell to advertisers. Cloud could be a lot more efficient (all told) for the former than the latter.
