Since evolution knows nothing, it would also not understand the consequences of a crowed planet.
This would have to be battled out in another fight of the fittest, what median age is the most beneficial for this planet.
As there is no regulatory force aside from battling it out in evolution, there should have been a long, outdrawn period where humans with a massive variety in lifespans lived on this planet and through the survival of the fittest, this age would come to be - and not because evolution was aware of the problems of a crowded planet.

Evolution is aware of nothing! This is the concept of Evolution, not intelligence or awareness of problems but mutations and pure chance governed until the survival of the fittest resolved an issue.

BUT NOT FOREKNOWLEDGE THAT A TOO HIGH LIFESPAN WOULD CREATE FUTURE PROBLEMS!