Again, I have to disagree. I think that any American downtown can use more variety to its spaces. It's one thing to "escape" to a quiet residential neighborhood, or to the mountains, but it's completely another to have lively pedestrian alleys in a dense urban context; and I think this is something downtown IS missing. It's only partly about getting away from the bustle of the main streets, it's largely also that these can be extremely charming and lively spaces - they're oftentimes some of my favorite kind urban spaces and are more pleasant than a busy city street. And I don't think it matters if it's visible from the street or not. If it's well done, it can easily draw plenty of people in, and being tucked away actually enhances the space.
That said, I was anticipating and hoping for some sort of plaza or square on this site as well, and was a bit surprised to see that this isn't what they are going for. That doesn't make it an urban design "fail" though, it's just a different approach. Seeing how little square-feet of public space they're actually required to provide, I'm not sure this would have been a large enough plaza anyway. A "pocket park" style plaza wouldn't have really been worth it, in my opinion. Denver needs something more like Union Square in SF, and we weren't ever realistically going to get that here.
Edit:
I don't want to beat a dead horse with a new post, but I was thinking some examples might be helpful here.
https://www.google.com/maps/@41.894282,1...3m4!1e1!3m2!1sLDjfwJQ-FfD75n8XHvB7RA!2e0
https://www.google.com/maps/@42.364386,-...3m4!1e1!3m2!1sUcLgQWZgK_usEFiLu7Beaw!2e0
https://www.google.com/maps/place/Boulde...!1s0x876b8d4e278dafd3:0xc8393b7ca01b8058
https://www.google.com/maps/@37.788285,-...3m4!1e1!3m2!1srq0v9vQqsvx_bVfHRmNAeg!2e0
I don't think these places are just historical anomalies, and they
certainly aren't dead, unlively, or "bad urban design." Quite the opposite, they are charming additions to the variety of the cityscape. And they don't always have to be the same either. We won't ever get a medieval alley, or narrow colonial street. So why not try it some other way?