Sure, it may have been interpreted on way but by no means that is the only interpretation. Good behavior doesn't mean lifetime to me.
Federalists papers talks about impeachment only way to remove. Yes, but that would apply even during an 18 year term and so would good behavior.
So, Barrett would certainly think and read that as law but others would not.
But, this too can be tested by a congressional Act as the number that established 9 jurists.
At least to me, and I don't count except at dinner that I am present, it is not spelled out as it could have been.