Monorepo vs not is not the relevant criteria. The difference is simply whether you plan your rollout to have no(/minimal) downtime, or not. Consider SQL schema migration to add a non-NULL column on a system that does continuous inserts.
Again, that's trivial if you use up and down servers. No downtime, and to your users, instant deployment across the entire application.
If you have a bajillion services and they're all doing their own thing with their own DB and you have to reconcile version across all of them and you don't have active/passive deployments, yes that will be a huge pain in the ass.
So just don't do that. There, problem solved. People need to stop doing micro services or even medium sized services. Make it one big ole monolith, maybe 2 monoliths for long running tasks.
Magical thinking about monorepos isn't going to make SQL migrations with backfill instantaneous and occur simultaneously with the downtime you have while you switch software versions. You're just not familiar with the topic, I guess. That's okay. Please just don't claim the problem doesn't exist.
And yes, it's often okay to ignore the problem for small sites that can tolerate the downtime.
