The issue with b-tree scaling isn't really the lookup performance issues, it is the index update time issues, which is why log structured merge trees were created.
EVENTUALLY, yes even read query performance also would degrade, but typically the insert / update load on a typical index is the first limiter.
If there is a natural key and updates are infrequent then table partitioning can help extend the capacity of a table almost indefinitely. There are limitations of course but even for non-insane time series workloads, Postgres with partitioned tables will work just fine.
EVENTUALLY, yes even read query performance also would degrade, but typically the insert / update load on a typical index is the first limiter.