TCP HOLB is hardly diminishing returns; it’s the defining way in which HTTP/2 is
not uniformly superior to HTTP/1.1—because of it, for a meaningful fraction of users, single-connection HTTP/2 behaves visibly worse than its primary competition, up-to-six-connection HTTP/1.1. (Lack of WebSockets support used to be another point, but that got fixed a year or two back.)
Some of the other benefits of HTTP/3 over HTTP/2, like the bypassing of TCP’s outmoded congestion control, are definitely more like diminishing returns.