CUBIC is a good baseline congestion controller. It's the default one that is being used in Linux, so it should provide adequate performance for a wide range of use-cases. Plus it's not so "olde" - the specification is actively improved (see https://github.com/NTAP/rfc8312bis), and the implementation of the algorithm inside s2n-quic even lead to the discovery of spec gaps that had been fixed as part of this effort.

Like most other QUIC libraries the congestion controller is also pluggable. So if a different one makes more sense for a particular use-case, it could be integrated in the future.

Is Swift (https://dl.acm.org/doi/pdf/10.1145/3387514.3406591) expected/designed to be used in non-intra dc environments where primarily quic is expected to have an advantage relative to tcp?

I agree that it still would be nice to have as an option.