I am having a hard time understanding you, BTC is not backed by tether, it is not backed by anything, but its active community that buys and sells it, gives it its value. Usdt is backed by the U.S dollars they claim to have in their reserves, and that is why it is pegged 1-to-1 with the U.S dollar, though i know there are so many usdt coins backed by thin air.
Tether is not being backed 1-to-1 with USD for a long time now, according to Tether company. It is pegged, until it isn't.
If you are holding BTC, then you are holding BTC, if you are holding usdt, then you are holding usdt, the failure of one does not affect the other.
Yes, number of coins you hold won't change - ever. But the price will and Tether collapsing will have indirectly the same effect as if more Bitcoin was printed (more than the scheduled emission).
Unless it will recover from that massive collapse which I already expressed my opinion about.
Another question regarding this scenario is, how many people will not panic sale and will be able to hold until it recovers - in result many will be affected as if it was inflation of coins itself.