Decimation served a few really good purposes for the Marvel Universe. First off, reducing the number of mutants prevented the creation of random new characters when there was a plethora of unused characters that are in comic limbo. Second it provided some great development to characters that were mostly being forgotten e.g. Jubilee and Moonstar. Lastly, if the mutants population didn't contract, more and more mutants would have been born and soon they would larger problem for the Marvel universe.
With mutants coming back, that means eventually the Marvel universe could face the same problems as it did a few years ago: too many mutants. In twenty-thirty years writers may do something similar, even if it is because of a lack of new ideas. It is not like there is a shortage of characters that could do it. There was already a witch hunt against Wiccan because he has the power to do it. Every time two superheroes have kids, the kid has to be overpowered, Franklin Richards is a great example. Not to mention a second Decimation in a couple decades would allow a lot of the older X-Men to be depowered and therefore retire and allow the younger X-Men a chance to finally take over.
Does anyone else foresee a second Decimation or Decimation like event occurring? Maybe a cure that is forced upon a large number of them, or another Legacy Virus. Killing off a lot of Mutants at once is not a new idea but Decimation did it at a grander scale.