Why are you diamond members so selfish?
If there are rooms available, don't you think it's fair to upgrade golds?
The best solution is to prioritise diamond upgrades.
Because by virtue of the higher requirements they deserve greater benefits. It's the same as many airlines give SWUs to the highest level but not to those who don't meet that requirement.
IME, international lounges do get very crowded and given the easy ability to attain Gold only makes this worse.