Brian;
I've been having a think about your question.
Wiki gives a pretty thorough description of the
theories about the end of the Universe. Most cover ideas about the fate of matter, given the data/observations presently at hand.
A summary list is:
1) Big Freeze or heat death
2) Big Rip
3) Big Crunch
4) Big Bounce
5) Multiverse: no complete end
6) False vacuum
7) Cosmic Uncertainty.
Each of these outcomes would seem to be valid, depending on the framework one chooses. The opposite would also seem to be true ie: some outcomes may not be valid, depending on the framework one chooses.
For example: the many-worlds interpretation of quantum mechanics posits that the destructive capability of decay from a false vacuum state to a true vacuum state, will not result in in the annihilation of matter, energy and spacetime (unlike the 'effective' destruction of matter (perhaps) in the Big Rip scenario).
So, a simple answer to your question might be that it would seem that the old adage: "that matter can be neither created nor destroyed", may not be necessarily so … and … it may be necessarily so … depending on which flavour of model one selects.
Unfortunately, the answer may not be as clear cut as one might hope for.
Other comments welcome.
Cheers