There’s a hidden MMR for BG that should never get reset, assuming the servers are working properly. I’m not sure if it decays or not, but I’m guessing not. Your visible MMR may or may not be the same as your hidden MMR depending on how much you play. The visible MMR will catch up to your hidden MMR if you get top fours.
I’m not sure how matchmaking works in BG, but I still face undead players and not-so-good players as I’m getting near 6000. If you lose a lot, you should run into players even worse than me eventually.
By the way, we should be getting a shakeup in BG soon. I think the .2 patch is supposed to come with the Naga tribe and a new keyword. New heroes will be there too for people with BG Perks.