The one thing this perspective--all these wonderful photos posted in this thread, from this exact vantage point--conveniently overlooks, of course, is this. A remarkably blighted structure, abandoned and given over to rot since at least 2010, if not prior. Of course, it does represent some exceptional redevelopment challenges, in terms of any would-be developer seeking to "go tall" there, to recoup purchase price and obtain sufficient ROI.
For what it's worth, the FOR SALE page for 51 High: https://greenway-bldg.com/. Per the City of Boston, owner is still John Walsh.
The history of that building is actually pretty interesting. Was a hardware store that the guy owned for decades. When he finally passed away, his wife ended up selling the building (the mid-2000s, I think?) for like $500k or something, and it's sat ever since. At one point they claimed to be getting a AAA high-end retailer to anchor it and has been on/off the market. I think they put it back on this year - last I checked years ago they wanted millions for it. Always imagine how great it would have been to get it for $500k and then rehab it into a storefront at ground level and one hell of a 4-story house + roof deck overlooking the Greenway above.