No, no, dude, you are totally missing the point. It's cheating to choose any point in time, whether it's 75 years or 750 years. Cutting things off at a point in time leaves an entire generation who paid in all their lives but get nothing out. Sure, if you get to do that, things aren't going to look so bad, but it's a lie. (Unless you're ditching the pension plan framing and going with the redistribution paradigm, in which case our entire conversation has been irrelevant.)

If you really insist on considering a more limited set, you remove people, not years. So you would pick a group of people, say everyone born before 2020, and work out the balance between their payments in and payments out.

(Do I need to explain what a Ponzi scheme is, and why it doesn't work?)