I get it. I can't say I'm a huge fan of Delta but being based in Atlanta and prioritizing a non-stop flight over all else....Delta and their partners by default gets most of my business. That said, I still fly AA/UA/TK/JAL/BA often enough. A couple items to consider:

Connecting in LHR T5 for Europe is painful. Granted, Atlanta doesn't have many options for Trans-Atlantic, but I'd seriously consider Lufthansa and connecting in Germany. Turkish is an option too if your travels are to Eastern/Southern Europe but not worth it for Western/Northern Europe. The Club all non-Delta/non-Delta partners use for int'l customers is the Club at Atlanta. It's terribly overcrowded during peak period when the int'l flights are leaving as not only does it have all of those PAX but also it is the only Priority Pass option in ATL (you'd have access with Amex Plat if you're not otherwise eligible with status or class of flight), the food is merely ok (usually worse compared to the SkyClubs), but alcohol selection is better. As you probably know, there is a Centurion Lounge being built out in Concourse E, but I have to imagine it will be overcrowded from Day 1, between Amex Plat and Delta Reserve cardholders.

BA Avios is far better as far as redemption value, but do you fly often enough to Europe to collect enough Avios to be worth it? Also, keep in mind BA adds $$ surcharges to redemptions on its own flight which can get very expensive. Or would you also switch Domestic flying to AA, knowing you'll have to connect to many locations served by a Delta non-stop. Even if AA has a non-stop is it worth it? My most frequent route is ATL-DCA and while AA has non-stops, it's far lower frequency than DL and many of the flights are on Regional partners that seem to be the first to cancel when any kind of weather event. AAdmirals Clubs, like SkyClubs are highly variable as to experience and food/drink quality.

Finally, as another poster said, if you're flying Paid J and have an Amex Plat, you really don't need status on any airline.