What you're doing by keeping up to date continuously is reducing the cost of a major version bump down the road -- should that become required.

For example: If you had a Rails 3 app, and you updated to Rails 4 when Rails 4 came out, then to Rails 5 when that came out, you'd find the upgrade to Rails 6 is relatively straightforward. If, however, you stayed on 3 until 6 came out, then discovered you _really really_ need to be on 6 for whatever reason... Well, that's a nightmare of a project for any non-trivial app.

Each individual upgrade takes time and effort, of course but the developers and/or community of whatever tool/library/framework are absolutely gonna chart an upgrade path from (X) to (X+1). Going from (X) to (X+N), however can be exponentially more complex -- and you'll rarely find clear guidance / documentation / etc, if only because of the large number of permutations.

Now, why might you find yourself in sudden need of a major upgrade? A common situation is that a major security vulnerability is discovered that affects you -- but the version you're on is past the end-of-life for security updates. That's not the only possible situation, but it's perhaps the most obvious and broadly applicable scenario.

The worst possible time to have to face a potentially very challenging upgrade process is when there's a zero-day in the wild, and you're vulnerable.

So really, what you're doing by keeping deps up-to-date is mitigating the risk of winding up in that situation. This comes at a non-zero cost, of course. Hence your question.

The difficulty is that it's really hard to put a meaningful probability on whether/when you might wind up in a situation like that. I've started a bunch of companies over the >20 years I've been coding professionally, so I've gotten to see this play out in a variety of ways. I've had companies where it was never an issue. I also had one where my last-ditch effort to save the company was foiled by an upgrade path that effectively amounted to "rebuild everything from scratch". That one wasn't security-related and it was a (3D) game, so the circumstances there are probably pretty atypical for most folks on HN -- but it was a serious kick in the teeth for me.

Given that, I tend to assume the probability that I'll be in a position where I _must_ do a major, multi-version upgrade goes to 1 on a timeline of about 4-5 years. From there, it's just a matter of how effectively I can amortize those incremental costs. Patch-level releases? Automate the update (e.g. DependaBot). Minor upgrades? Lean in on anything where changes are mandatory (pretty uncommon), but don't block things up on fixing every deprecation -- those can be addressed incrementally. Major updates? Deal with that on a case-by-case basis, factoring in how major/breaking the changes are, and what are the circumstances the business is facing. Often this involves doing a spike of a _super_ rough upgrade, just to see what breaks.

I can't actually think of any circumstances where I'd look at the costs of incrementally keeping up to date and conclude that it was a bad idea to do so, although I confess that may just represent a lack of imagination and/or a bit of paranoia on my part.