I couldn’t agree more on the importance of avoiding going through a vtable every time a TBP has to be called. This is an often overlooked but crucial point for adopting good object-oriented practices in high-performance codes where every nanosecond matters, e.g. in CFD, where (most) classes are almost never really extended.
A while ago I had attempted to foster a discussion on simplifying usage of non_overridable: I believe it is a low-hanging fruit for “relatively easy” standard extension that would bring significant benefits (no more classes clogged with non_overridable keywords)