As an outsider, I don't know. There is a presumption that since things have changed over time, they will continue to change. This very well might not be true. I didn't want to get too Kuhnian, so I just used the word "stagnant" for effect.

Once again, as an outsider it doesn't seem to me that they are anywhere near "done", but they sure as heck look like a mature science. Physics and its children have given us amazing things. Spending a lot of time playing with math wasn't one of them. All sciences have one aspect in common: until you get to reproducibility, the conversation in the community tends towards groupthink over time. That's a human characteristic not related to any one field of study.

Mature implies that no further major improvement is required. That does not seem to be the case with the foundations of physics given some of the fundementally unresolved inconsistencies.