As a counter-anecdote: multiple startup projects I've worked on with separate MySQL setups where each had just a single master + two slaves (one warm for fast failover in case of hardware failure or upgrades, one cold for slow analytics-style queries) did just fine with millions (to tens of millions) of users. No downtime at all for years on end.

MySQL and Postgres are massively more widely-used than Cockroach and Spanner, broadly very successfully. It's entirely feasible to run it with high uptime.