Fine, so you write a bunch of code, and release it under an NC license and a paid commercial license, no problem. Then someone comes along and submits a PR. What now? Will you accept their code and monetize it, despite the fact that you still deny that person the right to monetize your code, or will you refuse all PRs?
The problem with these proposals is that it creates an unequal relationship; some developers have more rights just because they started the project, even if others have contributed significantly.
I have a few things that I plan to release at some point. Small stuff, commercial use probably isn't a big issue, and I have no real plans for monetisation as this is hobby/interest stuff not the day job.
My intent is to go for full-on AGPLv3 and not accept PRs initially. If someone sees a bug they can tell me "if these options are picked there is an infinite loop" or "you've missed input sanitisation here" and I'll put together a fix for it. That way all the code is mine, I have full control, and I don't have to worry about treading on someone else's toes if I relicense later. When/if I later decide upon a more permissive license, or to try monetise through commercial licensing, or if I chose another route entirely, then changing will be frictionless.
In the meantime anyone wanting to work on it themselves because I'm too slow, they don't want to work my way, or "just because", are free to fork (assuming they are happy with AGPL), or contact me to negotiate other terms, or write something themselves from scratch to deal with the itch (again: these are not Big Things, I certainly don't expect to be releasing the NextBigThing™).
More likely it discourages others from contributing significantly. The PRs requiring copyright assignment end up being minor fixes, and anyone willing to develop a bigger functionality will just start their own project.
The problem with these proposals is that it creates an unequal relationship; some developers have more rights just because they started the project, even if others have contributed significantly.