I agree that the integrated amp has been elevated to a viable high-end product. That said most manufactures that make high-end integrated amps also produce more expensive, farther-up-the-chain separates.
Here is an excerpt from the Levinson site on some pros for the integrated amp:
Combining power and preamplifier functions into a single chassis is a well-established method of delivering high performance in a more convenient and cost-effective package. In addition to savings on chassis and shipping cartons, two interconnect cables and their associated connectors are eliminated. With preamp and power amp sections designed to work together, the preamp output buffer (found on separate preamplifiers so they can be used with a wide range of amplifiers) may be eliminated.
Here is an excerpt from Larry Greenhill in a Stereophile review.
Why is the integrated more economical? It's less expensive to build, having just one chassis and shipping carton...
and achieve further savings with a closed design that doesn't interface with other amplifiers or preamplifiers. This eliminates the circuits for a preamplifier output buffer and power-amp input buffer.
There are other savings. Packing a stereo preamplifier and two amplifier channels into one chassis saves space. Hum is reduced because amplifier and preamplifier now share exactly the same ground voltage, so less shielding is required.
Equipment clutter goes down, and the spouse acceptance factor goes up. This "sweet spot" of compact size and affordability keeps the integrated alive.
Most of us here know the importance of synergy in a system, with an integrated amp two (or three if you count the PC) variables are eliminated; those being, matching the amp and pre and also matching an extra interconnect.
Separates also have their pros, such as more flexibility and the chassis being separate give manufacturers more room to elaborate on their designs
I dont think we can categorically say there is a price point where an integrated is comparable to separates; there are too many variables and always exceptions. It simply comes down to two separate options to the same means.