I remember the xs750, it wasn't up to much compared to the 4 cylinder competition at the time, especially Suzuki's GS750. I do remember a girl who used to own one, though, and she was a much better ride than the bike!
The first modern Triumphs certainly borrowed more than just engine design from Kawasaki, the frame was "very heavily influenced" by the GPz900 frame, but not, of course, a rip off.
Despite Triumph and others going large with their engine capacity, the UK market is minor compared to the European mainland, where the smaller classes are much stronger sellers. The popularity of the Street3, Fazer 8 and Z750 there means it's only a matter of time before the manufacturers look at developing a mid-sized bike before the litre+ model. In fact, it looks like Triumph are already doing this, with updates to the Street3 before the Speed3.