Two comments about Poly:
1) About 15% of the Island Tower rooms are duo studios. I think it's reasonable to assume those 2-person rooms tend to skew toward singles and adult couples.
2) I can certainly see the new Island Tower pool appealing to families with younger kids. Yes, there is also a kids play area at the Volcano pool. But if you don't have use specifically for the slide, other aspects can be found elsewhere.
It's probably inaccurate to paint a picture along the lines of "OMG that's 250 more families who will all be at the Volcano pool!" Not everyone uses pools, certainly not on the same times and days. Not everyone values access to a slide. People who previously would have gone to the Volcano and Oasis pools will soon have a third option.