That literally makes no sense at all.
Sometimes, people die. Once they die, they’re no longer in control of things that happen, since dead people can’t take actions. Responsibility falls to the people who are still alive and still able to take actions. Granted, their actions can have a lingering effect, but it’s not the be-all-end-all that you’re making it out to be.
Someone used the example of the Founding Fathers earlier. Using that example, America is destroyed now, it will in some way signal that they failed to create a long lasting nation.
400+ years is long-lasting, though. Also, surely if America is destroyed, that has more to do with the forces that actually destroyed it than it does with people who’ve been dead for hundreds of years. The idea that The Founding Fathers are to blame is an extremely weak technicality.
If Twilight failed to bring long lasting friendship to Equestria and actually indirectly ruined Equestria so much that ponies went back to being just as bigoted as they were Equestria’s founding, she failed.
If Twilight failed, she failed. That’s just using circular reasoning without establishing a concrete fail-state. How long is “long-lasting”? If her Age of Friendship lasted hundreds of years, then surely she succeeded at bringing “long lasting friendship”. Also, by your own logic, surely it was The Founders of Equestria who failed, since it was their union that didn’t stand the test of time, not Twilight’s.
Also also, you didn’t actually address the point. I’ll take that to mean you do unironically believe that “Either your work lasts forever, or everything you did is meaningless. Those are the only options”. Which is just you projecting your own nihilistic worldview onto a horse cartoon for children.