I'm curious why the UA-only focus.

For most European destinations that are multiple carriers offering service and it may well be that a different carrier offers a better route, product, and price. Buying tickets for others in the recent past (i.e., status not a consideration), I've found that AF and DL offered the best combination (particularly with DL still offering 2 across in Y on many aircraft). AF in particular tends to offer very good award availability and the miles are easily obtained through credit card transfers.