If you are flying enough to reach PM (even if that does involve longer routings for extra miles etc..), you should still be able to make FO/silver pretty easily, assuming that Delta is cheapest for some of your travel.
I've been doing this, and base my ticket purchases on price and (sometimes) schedule, with airline or alliance as a tiebreaker. Even still, I've managed to get silver on UA, and might be able to get silver on AS this year.