(or maybe just bitcoind) would be quickly out-competed if that rule was adopted.
That is the point. Development would not stop. Bitcoin would not be stopped. Only a certain (and currently central) sect of it will and it would only apply to protocol changes, not client development.
You're contradicting yourself!
I will also add that recently a bug was found in Bitcoin-Qt's latest build (Ultraprune) that changed the Bitcoin protocol, inadvertently.
Ultraprune did not intend to change the protocol. To avoid bugs like this entirely,
all core client development would have to stop. That means at the very least, blockchain download could never be made to scale.
I'd suggest helping find and fix these bugs, but you've made it clear you don't even have skills enough to be a competent tester.
P.S. Why five years? So new competing releases have time to develop and challenge any dangerous changes that come our way.
Furthermore, competing clients are inherently vulnerable to the same kind of potential bugs. To achieve a guaranteed protocol freeze immune to any bugs, you would have to forbid new clients!