Interesting discussion.... and a great read. Personally I believe ITB is totally doable these days assuming one has captured good audio on the way in. One thing I didn't see in the discussion was comparison of a professional hardware studio to a professionally crafted ITB audio chain that follows the same audio flow as a hardware-only based studio.
Something to consider when mixing ITB is to create the entire hardware chain used in a pro studio with your plugin tools, not just adding random plugins or presets and expecting your mix to magically sound professional. Spent many years and $$$ buying plugins with little progress in quality and little to show for it:(
For me, only when someone suggested starting with a basic framework to truly emulate all the components that make up the professional hardware studio's "audio magic" did it start to come together. i.e, start with a proper Channel Strip chains(tape, compressor, console emulation, etc), Bus Templates (compression, delay, reverb, etc), Master Bus settings (Tape, Console Emulation, Compressor, Limiter, etc...),
that are all properly gain staged, so your mix starts from a solid foundation. If you don't have that initial foundation to start from, all the great plugins you find out there will never sound as good as the video demo that prompted their purchase. I think what is missing in many ITB mixes is that something is missing in the overall chain or a bad initial capture of the source material occurred.
When I first bought a UAD Apollo and some UAD plugs last year and started inserting random plugs into my mixes, my mixes were not incrementally better as a result. After watching a video on the EMT Plate 140, understanding it, and using the plug in a digital realm did not automatically produce the same results as how they were used the EMT Plate in a studio back in the days of tape and mixing desks. However, set-up the EMT Plate Reverb Bus to include some Studer or other Tape Sim, Compressor, EQ'ing and then it comes pretty close to the original hardware results in my opinion. This is of course assuming a good initial capture, proper gain staging, etc... My 2 cents...
Regards