If the NFLPA wants grass then they are going to have to share the cost with the NFL, and it won't be insignificant.
Putting down grass in a place like Ford Field isn't impossible, just like growing plants without direct sunlight isn't impossible. It's just super expensive.
Need to separate outdoor vs indoor ones. As of this season, of the 30 stadiums that host the NFL we have a 15/15 split (but 17/32 teams have turf since LA/NY share)
Grass we have:
SF, AZ, CHI, GB, TB, PHI, WAS, KC, LV, DEN, Pit, CLE, BAL, JAX, MIA
Turf we have:
LA, SEA, MIN, DET, NO, ATL, CAR, DAL, NY, CIN, IND, HOU, TEN, BUF, NE,
Of the turf, 8 indoor and 7 outdoor.
Houston, an indoor one has had grass before, so for them it's possible.
You can cut up a grass field into a few large sections or many smaller sections. Question is where and how do you store it to maintain the integrity of the grass. And what is the timeline needed to make the changeover and how long does it need to sit after it is put back into the stadium before it is game ready?
Tottenheim and Man U. have stadiums now that can do it automatically to remove the grass field. But, haven't seen how long the field needs to sit once returned to the surface.
That's the challenge on top of the higher costs. These billionaire owners can handle covering the few extra millions it costs to maintain the grass.
But, it's the other events that they need to change for that is a concern. Seattle is packed with 25 concerts/shows and 5 soccer matches during the Seahawks season. I'm sure most of the indoor turf ones are booked like that as well.
Only the colder midwest to eastern ones are less packed. Cincy, NY, Buf, nothing but football during the NFL season. NE, has 3 concerts and 4 soccer matches or something. But, they need to account for MLS playoffs.
How can you maintain the integrity of a grass surface and still hold other events? That's the main question to figure out. What is it like in Europe when they host concerts and European football matches.