Fundamentally, you are correct and I will agree with you. But have you tried any of the higher end digitals from the latest generation of pianos, e.g. Kawai CA79/99 or Yamaha CLP-745/775/785? Those come with some pretty serious audio setups—with 4-6 speakers placed in different locations—that can really pack a punch with the high quality recorded samples from concert grands. As someone who regularly practices in his apartment on his digital and plays on acoustics with my piano teacher once a week in a small piano studio, I can confidently say that these higher quality digitals are nearly indistinguishable in terms of sound richness from those acoustics. (I'll be fair though, I think those acoustics leave quite a lot to be desired and aren't really in the best environments, but I would bet that they cost more than what I paid for my CA79.)

Digitals will always be outclassed by a regularly-tuned decent quality acoustic that is well-placed in a room with decent acoustic characteristics, sure. However, modern higher-end digitals raise the bar quite a lot that, in my opinion, it makes the value proposition of, say, a Yahama U1 a little more questionable for the vast majority of people who aren't professionals or aren't able to set up an optimal acoustic environment at home.

(Worth also mentioning, the key actions on higher-end modern digitals are also pretty damn good these days. My CA79 does a pretty good job at allowing me to play a light legato, and I felt like that experience was quite transferable to some pretty nice Yamaha and Kawai grands I was recently able to play around with. Now, I'm pretty sure I may begin to notice the limits when I start working through level 9/10 (or beyond) pieces in a few years, but that's the point that I'll know it's time to buy a home and put in a grand piano.)