There have been rapid shifts in the some of the things that AV receivers do and the biggest driver of that shift is the increasingly complex encoding that have been adopted to make movies more immersive. That means that systems that were not capable of decoding discrete 5.1 sound tracks have been left in the dust compared those that do. The evolution of 7.1 and now object oriented sound tracks are further big advances.

The other factor is that the Digital-to-Analog chipsets evolve at about the pace of other computer technology -- that means not just better resolution of higher bit rate audio but improvements to things like the overall signal-to-noise levels, greatly enhanced resistance to artifacts of the digital conversion, and more features at lower prices. Those features and dropping prices makes it easy to incorporate things that once exotic like parametric eq. and similar enhancements.

Meanwhile the other things that receivers do, like tuning into broadcasts / internet sources, switching various components and providing amplification to drive speakers, has not really evolved nearly so much -- the start of the art for amplification arguably remains high current balanced designed like those offered by Outlaw and other firms. I know all about the topologies that are possible http://www.audioholics.com/audio-amplifier/amplifier-classes but the fact is for lots of reasons the differential drive amplifiers are hard to beat -- http://www.ati-amp.com/differential_drive_amplifier.php
You can argue that some streaming sources have proven to be more responsive to offering high bit rate content, but given how many people are attached to vinyl or CDs that is not a particularly compelling reason to upgrade.
The last part of the function of AV receivers, as relates to component switching, was once mundane but now complicated by things like the shifting standards for HDMI 2.#x and associated HDCP x.x and the associated headaches of 4k scaling / pass-thru. If I had to lay money on what "final tweaks" are giving the Outlaw team the most trouble it is almost certainly in that realm...

All that aside, the decision to "go with separates" is less about the dollars and cents of TODAY than the longer term satisfaction and flexibility -- for folks that will likely never care about swapping out any of their existing components it might not make sense to fret about the differences between separates and a receiver. For nearly everybody else, who likely will swap out amps, or displays, or speakers, or sources or perhaps even create multiple entertainment zones in their home the value of separates is much clearer -- you simply have more choices. Together with the boost to performance that one gets as the codecs and DAC advance as well as just the "Christmas morning" effect that many of us appreciate from upgrading the pluses of separates don't really need further justification.