A lot of the expansion is demographic, baby boomers are getting older and with that results in more physical disability. When they start reaching retirement age they will transfer over to elderly benefits and disability numbers will go down. Aside from that if SSDI is what's keeping a certain subset of workers displaced by automation afloat, I fail to see what the issue with that is is. It's called "social security" after all, and even if it's not meant to protect against technological and economic change, perhaps it's time we reconsider such a view. Make it sort of a bridge to a possible UBI in the long term.

In terms of people "gaming the system" that really depends on your perspective. Some people are on disability for mental disorders like depression and anxiety (among many other things), which a lot of people don't consider real or debilitating. If that's how you feel than yeah, you would be accurate to believe the system is being gamed. What's not happening however are mentally healthy and able bodied individuals drawing monthly checks. I mean I'm sure it's happening somewhere but it's incredibly rare.