I didn't think buying more points kept them from expiring. It's not based on activity unless that's a recent change.
There are two separate expirations at WyndhamRewards. The one at 18 months is avoided by any activity. The one at 4 years has no way to avoid it.
So it depends on which expiration you're talking about as to whether activity extends it or not.
Thus please always specify 18-month or 4-year expiration, rather than just saying "expiration", in the context of WyndhamRewards.