Seized? Probably not. How about converted? Into long-term treasuries? For "safety," for our own good, for the Good of the Nation?
Mandatory conversion to Treasury securities? Ok, I'll say it's not impossible. Some pension plans already are. Credit unions by regulation have to hold assets in "safe" securities, which generally means T-notes. But only universal compulsory T-note retirement accounts? Eh, I'm not seeing that.
So, my guess on that would be "possible, but unlikely". Again, old people. They are the overwhelming largest chunk of federal spending, and will be for quite some time. Expect any "solution" to soak young and middle age workers. You know, exactly like now.
Note, I'm not saying the current group of old folks caused this situation. They did pay in, and I respect that. I also know it's not going to last until my retirement. I respect that part as well. It's a situation with no magic solution. The closest is "least worst" solutions, and there are few of them.