I think this idea would be somewhat problematic.
For one, what about any violations that might be currently in your statistics? If I have a Level 2 or 3 violation preventing me from accessing the Expert Server, why would I want to freeze my statistics? Might they not apply to freezing your statistics or would there be something else in regards to that?
For two, the current system, where progress towards your grade can be lost, is like this for a reason. Staying active as a Grade 3, 4, or 5 demonstrates your amount of dedication, and in most cases your expertise, and may become somewhat meaningless should you be able to freeze your statistics.
I don’t mean to come off as condescending or rude, but there are some fallacies to this request in particular that I’d like to point out.