> How many internal libraries does your "separate services" contain? You service A depends on library alpha@1, your service B depends on library alpha@2. All happy now. Introduce another layer, your service A depends on library alpha@1, beta@1, and alpha@1 depends on gamma@1, beta@1 depends on gamma@2, what to do now? It does not even matter how many services you have now.
Got several thoughts on this one. First, lets look at how bad the issue really is:
To start using beta@1 you need to upgrade alpha@1 to alpha@2 that depends on gamma@2. What's the problem with that?
The same situation can arise with 3rd party dependencies, except there its much worse: you have zero control over those. Here you do have the control.
Now lets look at what this situation looks like in a monorepo: you can't even introduce gamma@2 and make beta@1 at all without
1. upgrading alpha@1 to alpha@2
2. upgrading all services that depend on alpha@2
3. upgrading all libraries that depend on gamma@2
4. upgrading all services that depend on gamma@2, if any
So you might even estimate that the cost of developing beta@2 is not worth it at all. Instead of quasi-dependency-hell ("quasi" because your company still controlls all those libraries and has power to fix the issue unlike real dependency hell) you have a real stagnation hell due to a thousand papercuts
My second comment is about building deep "layers" of internal dependencies - I would recommend avoiding it for as long as possible. Not just because of versioning, but because that itself causes stagnation. The more things depend on a piece of code, the harder it is to manage it effectively or to make any changes to it. The deeper the dependency tree is, the harder it is to reason about the effect of changes. So you better be very certain about their design / API surface and abstraction before building such dependencies yourself.
Major version bumps of foundational library dependencies is an indication that you originally had the wrong abstraction. No matter how you organize your code in your repos, its going to be a problem. (Incidentally, this is also why despite the flexibility of node_modules, we still have JS fatigue. At least with internal dependencies we can work to avoid such churn.) It should still be easier with separate services, however, as you can do it more gradually.
Last note on left-pad and similar libraries. They are a different beast. They have a clear scope, small size and most importantly, zero probability of needing any interface changes (very low probability of any code changes as well). That makes them a less risky proposition (assuming of course they cannot be deleted)
Those are my (hopefully nuanced) 2 cents.