Foley wanted to play at Allegiant with MLS and when public funding was being discussed to build it the suitability for MLS also was used as an extra selling point, I think David Beckham even came here and talked to lawmakers.
I think they would convert UNLV's turf to grass, plenty of the top college football teams play on grass (UNLV football isn't valuable at all anyway) and it tends to be easier on joints. The MLS team would probably be on the hook for the upkeep of that new grass year round though.
The MLS overlap with the NFL season is between August pre-season to early October. So, they can deal with the grass when their season begins in the spring through to the end of the summer.
With grass fields, if you remove them in section and then reassemble them after an event, how long does the grass need to sit before it is suitable for play for a soccer match or football game?
Where the NFL played in England at Tottenheim, they retracted the grass field into 3 section to play on an artificial surface. I recall that there was an article that said they could play soccer on the grass once it was returned to the surface in around 48 hours.
But that is grass in 3 large sections. If you are taking it up in like 20-30 sections since current stadiums are not designed to do it mechanically like Tottenheim, how much more time does that add before a game is played?