From Wikipedia, the Guggenheim can apparently show about 6% of it's collection at any one point. Seems like there's a fairly obvious way to generate more money there. But no, it's the union's fault they have money problems.
If you're interested, see https://news.artnet.com/art-world/brauer-museum-controversia... for a recent example.
I bet they could even lease out a fair amount of it with requirements of maintenance and some amount of public accessibility. That alone would drive down their costs at least.
There's more art of value than there is space to show it, so we rotate stuff in and out with special exhibitions and whatnot.
They're restored, conserved, and rotated in and out of the museum's public collection so it's ever-changing, which helps generate repeat business.
(They're often also available for academic purposes.)
There's some truth to helping generate repeat business, but even with a 100% yearly rotation rate that's a 17 year cycle to see everything without counting new additions. I doubt people are going to lose interest because two years prior to the new showing they could have traveled to Oklahoma City to view it.