I share your thoughts on the DPR review, Simon. I guess we need to allow that this was one review of one example of the 90D, but if the assessment holds up in other reviews it will indeed be disappointing that Canon was not able to improve the through-viewfinder servo AF performance given the extra functionality they've put into the AF system.
For bird photography using Canon's APS-C SLRs this may leave current users of the 80D and 7D Mark II with no clear upgrade path except to move into full-frame (whether it be current DSLRs or potential future R series bodies). But, if your main birding lens is the 100-400 mm L II zoom, my experience of this lens with a full-frame camera is that 400 mm is just too short a maximum focal length for bird photography. So, you tend to be compelled to use the zoom with the 1.4x TC, a 560 mm f/8 combination which on the 5D Mark IV, excellent camera though it is, has some autofocus speed issues with that lens combination.
In short, unless you want to make the very large outlay to step up in Canon's lens line to the top-end telephotos (which enable focal lengths including 560 mm at f/5.6, 600 mm at f/4, or 700 mm at f/5.6), I'm fairly convinced the Canon APS-C sensor camera has a place in bird photography - but ideally with something like the 90D sensor in a camera body representing what the 7D Mark III might legitimately have been expected to achieve in regard to improved AF performance (both speed and accuracy).
One thought in closing - the Canon R series sports/action camera rumoured to be in development is reportedly not going to appear until 2021, which given that current 80D and 7D II owners may get itchy feet in the meantime, may put some pressure on Canon to bring out a 7D III upgrade at some stage in the meantime. One interesting aside to this is a clear statement attributed to Canon USA and reported in the Digital Picture preview of the 90D that I posted above, that the 90D is not a replacement for the 7D Mark II. Intriguingly, this contradicts most other media opinions about what the 90D represents.
Cheers, David