Yeah, this is what I keep thinking. For all the nostalgia for the grass, I really don't remember people using it much. If I ever sat down in that grass, I don't remember it. I poked through Street View and they seemed to favor taking pictures on sunny, busy days, but I couldn't find more than single-digit number of people on the grass. There's the one photo that keeps getting posted to show how busy the grass used to be. That's more people than I remember regularly being there, and even that is like a dozen people (with a telephoto lens that makes people look more packed together). And that's ignoring the ~6 months a year when it'd be too cold or muddy to even consider lounging in the grass. I think anyone who wanted to hang out on grass would go the Public Garden or Esplanade. Copley was never really that kind of place.
That's how I remember it, anyways. If anyone regularly used that grass, I'd love to hear what you did there.
The grass always seemed more ornamental. That is also a good thing, but then I also don't care as much if the grassy area is now smaller/moved/split up/whatever. It can be ornamental in a new corner of the plaza, and still serve the single-digit number of people who want to sit there.