OT, but I do. I think most of the criticism of OneKey comes from those of us who were moved there from the hotels.com reward programme which was massively more generous so it's just a big devaluation. A bit as if BAEC and IB decided to merge and suddenly, rewards were twice less than before for some of us.

I think the rest of the argument you make can be summarised as "surely, the rewards programme is not what matters". Many people make exactly the same argument about airline FFPs but ultimately many people still care about them as amply evidenced by this particular website. It doesn't mean that nobody do the "I will completely ignore FFPs and just book whatever is cheapest" (plenty of people do that) nor that they are wrong, but just that different people have different preferences.

As for the specific case of OTA reward programmes, I think they were never really primarily focused on flight bookings which tend to attract low rewards, but primarily on hotels, package deals and incidentals. So on that front, when you move from 10% or 15% reward value to about 3 or 4%, that's a steep loss though it is fair to say that the downward trend is not limited to the Expedia group here.