[yyqux]

Asymptotically, sure, you're right. Constant factors are often important in practice, and simple cost models (e.g. ones that don't model cache locality) will no longer give you a decent estimate of constant-factor differences in performance between algorithms.

I think the issue here is that, in the past, with shallower cache hierarchies, models that assumed a constant cost per memory access would maybe be off by smallish factor (I don't know, maybe 50%).

However, now memory access is frequently the limiting factor for an algorithm, and there can easily be an order of magnitude in variation between the average memory access latency for different algorithms (i.e. cache-smart versus cache-dumb).