I guess "the Murph" is being demolished, since the Chargers are moving to LA ?
Why do they need to demo it ?
San Diego U Tereros (a small, but D-1 non-scholly Pioneer league team must play SOMEWHERE. I can't imagine that it has much of a seating capacity though.
Petco Park (where Padres play baseball) may have issues. I'm sure the Padres don't want their field torn up. The mound and goal posts would have to come and go if they switch back and forth during September. And SOME baseball fields can barely accommodate a football field. Which is 160 feet wide and 360 feet long. A few years ago, Notre Dame played Northwestern (?) at Wrigley Field, and the brick outfield wall was RIGHT behind the end line. The coaches and refs thought it was REALLY unsafe, so every time an offense was threatening to score, the ball had to be moved to the other end of the field, where they didn't have this hazard.
San Diego State doesn't need 60,000 seats. Major League Soccer doesn't need that many. More to the point, they need luxury suites; that's how old the Murph is.
Not to mention the Murph was kind of built as a baseball/football hybrid. Even soccer looks a little small in that stadium. The field level at the Murph is frankly not intimate... oversized. Nothing at all like Wrigley.
So the structural alignment has to change and suites have to be included. The maximum proposed capacity, at this point: 30,000 seats. It might end up as small as 20,000 (if proposed MLS San Diego doesn't really feel like they can "compete" with Tijuana FC across the border). Cheaper to rebuild than to modify.