For my part, I've worked with two long-lived monorepos so far. Both were quite small, all told, but, nonetheless, in both cases I saw the monorepo largely serving as an incubator for bit rot.

All those direct interdependencies, combined with the impossibility of just using package versioning to allow a library's consumers to upgrade at their own pace, meant that developers had a strong disincentive to do any cleanup that might result in a breaking change. They'd tend to prefer patches instead. So the long-term trend was for everything to eventually become a tangled mass of patchwork.

Google has entire teams devoted to combatting this problem. I'm not sure most companies can (or want to) afford that.

This is one of those spots where I think it's wise to be cautious about blog-driven development. By analogy, I've seen experienced woodworkers have fun with mocking the DIY furniture projects that come up in places like Pinterest. One common criticism was that, since wood expands and contracts with changing humidity, but only in certain directions, if you put it together in certain ways, you end up with a product that will literally tear itself apart over time. Of course, this isn't apparent from the Instructable article, because it's a slow process, and all those pictures were taken when the piece was still brand spanking new. It's also not something you're likely to see written in the comments section or anything like that - the people I've known who know this sort of thing aren't generally the type to write Instructables articles or maintain their own DIY blogs.

I suspect something like this also happens with any decent sized codebase: Code is a living thing that expands and contracts with changing business requirements. Over time, it will twist itself out of shape. Weak coupling and fragmented codebases are two different (though related) ways of trying to limit the damage from this effect. They both bring their own challenges, though, and aren't perfect. Monorepos don't have the same drawbacks, but that doesn't mean they don't have any. One, I think, is that you're left directly exposed to the full strength of the forces that cause bit rot.

If you're disciplined enough to recognize this and treat it as another incentive to attack bit rot head-on, then you'll probably end up in a better place for it. If you think monorepos are a panacea, though, you're setting yourself up for the kinds of failure that people generally don't like to blog about.