[Matt Q]

I guess I'd say: write because you find it rewarding and let your successes be a bonus.

Besides, the world will likely have fallen apart in less than 200 years time as so much more of it become uninhabitable, crops and economies fail, terrestrial and marine ecosystems collapse, wars rage and millions of refugees try to find somewhere cooler to live and so on. People will likely have more pressing things to think about than poetry.

But yes, if it's that important to you that there's at least some small chance that you'll write poetry that will still be read and admired in 200+ years, and even be considered better than Shakespeare's, and you believe that no poetry written now can achieve that, then stopping makes sense.