Glad to hear people enjoy the series of Monroe tracks. I originally made them for someone else but figured there were enough folks interested in bluegrass here to make it worth sharing them.
I don't think many people feel that the music should be stagnant. As we've discussed in other threads, Monroe and others have experimented with different instrumentation and arrangements. The Grand Ole Opry used to have a resident snare drummer and any performer---yes even Mother Maybelle---that didn't carry drums was required to suffer the drummer to stand behind them adding a brushed snare fill to everything.
The question isn't so much one of limiting instrumentation but of deciding whether it still remains in the original genre or has become something different. Muddy Waters used to sing "The blues had a baby and they named it rock & roll." They didn't call it blues junior or newblues, it was something different. I've seen working bluegrass groups use a washboard but they've been urban bands serving an urban audience. Those bands also tended to be somewhat "looser" than one would expect of a more traditional bluegrass band. I confess I've never heard a bluegrass band with a musical saw. I'm guessing the saw would have great difficulty in maintaining the rhythms required in bluegrass but then I am neither a sawyer nor a saw player.