If it makes you feel better, I have had the same issue. When I brew any IPA, I drink IPA until it is gone. I saved 6 from an IPA I brewed for the NHC comp. I tried it last week and it was not very good. I purged everything, bottled from the keg, and it didn't seem to be much better than when I just bottle conditioned. Dry hops fade fast. The more I dry hop the more O2 issues I have. IPA beers just need to be hammered fresh. Buying a tank and regulator wouldn't set you back much. Purging is worth it IMO.