And if an app/package did grow to the point it needed its own repository, all the packages depending on it just need to update their dependency configuration to the new source.
The most typical case of this happening I've seen is an ancient unsupported legacy app. You still need to deploy it, but you only build it every few months or so. You rarely touch the thing, and its dependencies are pinned to old versions. The lack of support tends to drag down the rest of the monorepo, so it'll sometimes get "kicked out" as a bad citizen.
Really only reserved for the worst cases.