The NFL is the one with the issues at Sam Boyd Stadium. Unlike StubHub Center, Sam Boyd Stadium is old, the sidelines are small, facilities are run down and below NFL standards.
Nothing to do with the size of the venue. Lockerrooms, press boxes, etc. StubHub Center is/was a first class facility when it was built. Just smaller for MLS. Sam Boyd Stadium isn't and hasn't been updated. Now, Nevada's stadium in Reno has just recently been fully renovated. So, that's why it's a potential option.
But, since they're apparently keeping all their offices, etc in the bay area, moving to Las Vegas would be tougher than Reno. Reno is also a bit closer for the current Oakland fans to make the drive if they so chose to do.
49ers built Levi's Stadium with sharing it in mind. The Raiders have flat out refused to be the Jets in Giants Stadium. They want their own market and stadium and that won't work in Santa Clara. But, nowhere else around there is good enough for them, or outside 49ers market territory.