Also, the discussions are there because it's been more than a decade and the options have evolved over time.
Submodules are a bit clunky but the problem it solves is itself clunky. Bringing in another tool doesn't really feel like its going to reduce the burden.
I have yet to be in a situation where I blindly want to update all submodules. It is a conscious action, X has updated and I want to bring that change(s) in.
cd submodule, update, test, commit.
I haven't seen anything in this thread that really motivates me to learn another bespoke tool just for this. I'm sure it varies for different projects though.
Fast forward 15 years and see how the tooling this thread has been evolved and how many different tools people will have used and compare that to the stackoverflow post. I'm more inclined to invest time in git itself.