For destinations where NW metal flies, you can connect via DTW. For destinations not served by NW metal, there are always the KL-operated NW codeshares via AMS to pretty much everywhere in Europe (KL flies IAD-AMS daily). Plus with SkyTeam there are a bunch more one-stop options from WAS on AF, AZ, and CO.

I agree, however, that the OP should continue to consider UA. I'd ultimately say that if the OP's priorities are nonstop domestic flights and/or int'l upgrade opportunities, UA is the winner. If it comes down to having more routing options, more fare competition, and/or better domestic upgrade opportunities, NW wins. See also the other thread linked from the OP's first post here.