LH is starting PDX-FRA (non-stop) service. This might make Star very attractive for you.
DL used to be a fantastic airline in the 80s in terms of management practices (for both employees and customers) ... this devotion to quality caused me to go far out of my way to fly DL over the years ... but the past 5 years has seen a steady change, towards "industry standards" and DL isn't special any more. Skyteam is a joke of an alliance, so I have mostly switched to Oneworld and am now PM-equivalent on AA and QF; now fly DL when convenient, and might make PM again this year despite DL not being my carrier of choice (it is still a good airline, in terms of plane quality). But I didn't leave DL, rather DL left me (and I hung in there for several years ... the problems at DL predate 2001 for the most part).
I will say that AA offers far better service than DL on some routes, and the alliance benefits are far better than for Skyteam (also awards are actually redeemable). My QF Plat status gives me free F lounge access even when flying AA domestically (DL doesn't have anything close to the F lounge quality) so I won't miss the CRC if I do lose access.