I don't think that they will abandon LH at all. Lufthansa offers a lot of very important routes to them that BMI cannot give them. Especially routes to Western Asia and Africa are very vital to them. Plus a lot of more eastern europe destinations are served by Lufthansa and not by BMI at all. United has more flights to LHR than FRA so it could offer more valuable connections to cities in western europe such as Madrid, Nice, Dublin, pus the code-share posibilities within the UK. However, Luthansa is still much bigger than BMI and can offer United more (Don't forget all their US-Germany routes too).
Why is each American carrier only allowed one major European partner?
What's stopping Delta from having stronger ties with Alitalia? American with Iberia? While I know that UA hasn't made the strongest ties with SAS or Austrian, they theoretically could, and should.
BMI and Lufthansa are partners, so why would LH have a problemw with UA-BMI codesharing. I think it'll definately work out for the best.