You have to consider that an in-kernel implementation also hinders innovation - for example, things like BBR instead of Cubic/NewReno can be rolled out easily i

You have to consider that an in-kernel implementation also hinders innovation - for example, things like BBR instead of Cubic/NewReno can be rolled out easily in userspace through the application's QUIC library while it takes a while until all clients and servers a) use a recent kernel which has TCP BBR support and b) have it configured by default.