Whether or not the system is built on top of a framework is, in my opinion, largely irrelevant. Or perhaps it would be better to say that "it is impossible to guess if and how it would've mattered in the big picture".
Call me cynical, but dependencies are never free. Each adds weight to the codebase, requires upkeep and maintenance (sometimes including major updates or rewrites), and increases potential attack surface. Also many dependencies — and especially frameworks — bring in their own way of doing things, and since ProcessWire's architecture (hooks, data structure, etc.) is really a major selling point for the system, using a framework could've had a massive impact on that.
Part of the reason why ProcessWire is such a brilliant system could very well be due to it not being powered by a framework itself. This means that every decision could be made without considering how that would fit the framework.
What is also interesting, in my openiin, is the framework landscape itself. Back when ProcessWire 2.0 was launched, Laravel — now considered the latest state of the art PHP framework — wasn't even around yet. Zend Framework was probably the biggest name back then, or at least the one that most serious developers considered "most likely to thrive and stay around"... yet it was discontinued. Had ProcessWire gone with the "safe bet", that would've meant a big rewrite later, or alternatively staying on a platform that is merely a shadow of its former glory.
Long story short, I think Ryan made a good choice ?
Don't get me wrong, though: I'm a big fan of the Composer ecosystem — almost all of my modules are available as Composer dependencies, etc. And, in my opinion, ProcessWire actually integrates quite nicely with external dependencies. Since HTTP clients were mentioned, I'll use that as an example as well: in some recent projects I've used Guzzle together with ProcessWire. I may not be able to (easily) swap the client used by the core itself, but I've yet to come across such a need anyway.
Of course this comes with a cost: I recently had to migrate one project to a new server simply because a third party dependency decided to drop PHP 7.4 support. In a minor version, because why the heck not. Not that it's a bad thing necessarily to bump up the PHP version, but having to do that sort of thing in a hurry just because I prefer to use a recent and patched version of a dependency is just bollocks.
This is starting to get a bit rambly, so I'll just leave it here.