Agreed your jaw muscles are - muscles. Most people at an older age aren't able to do the splits. But with practice they eventually get to that point. Same premise for trismus. Physio is a tool you can't really do it forever in office as it's crazy expensive, but learning certain things to help improve stretch will overtime improve the situation. But it's all about diligence. I've been doing it daily since the beginning of treatment. Daily I stretch my jaw and mouth several times.
Hugs