You're probably right -- after all, you go to Germany a couple times a year. I've worked in France for the last year, so what would I know?...
You're quite wrong about AA doing worse to Germany because of BA... AA lost Germany when UA and LH joined up around 1994, well before Star Alliance being formed in 1997, 3 years before oneworld was formed, and 13 years before JV with BA was finally launched. Before UA and LH partnered, AA was the one codesharing on LH, and they had service to DUS, STR, MUC, TXL, and FRA. Once LH shifted to UA as their preferred partner, all but FRA wound up with 30-40% load factors. I remember being one of about 60 people on a TXL-ORD flight...
Germany is a larger economy, but it's also a fragmented market. Traffic is split amongst FRA, MUC, BER and DUS.
Connecting traffic is pretty well split in half between MUC and FRA, but Air Berlin is gaining some ground over DUS. Yet to be seen how BER will work out...
For DL and AA, anything they carry to Germany is likely terminating traffic. There's very little reason for either one to transfer customers to/from LH at MUC or FRA. The problem with that is that BER, FRA and DUS are all between 4M and 5M people. That's the size of Houston or Phoenix, give or take a couple hundred thousand. And BER is only 3M people (which makes the notion of AA running their own 773's there just that much more ludicrous).
If it weren't for the connecting flows from LH, you wouldn't see UA offering as much service as they do. I'd guess around 60% of the traffic at MUC and FRA is connecting.
London? Yep, UA's still flying a lot of metal into LHR. It's the busiest airport in Europe, and with LGW, accounted for over 100M passengers last year. FRA accounted for ~56M.
London has a metro population of about 8M people, and is the banking capitol of the world. For business, it has far more ties to the US than Germany does. Unlike Germany, I'd guess >70% of the US-UK traffic really is heading to London.
I'll see if I can pull the numbers to show the relative O&D markets vs. flow traffic at LHR, FRA, and MUC. Either way, you can't just look at GDP and decide there's a lot of demand. And we certainly know that load factors have nothing to do with profitability.