I have re-read all the pros and cons to an LH/B6 alliance and I still do not see this investment as anything other than that, an investment.
Yes, B6 could morph over time into a more full service airline but, I highly doubt it. Yes, F/J pax might be willing to overlook the transition from a great long-haul flight in F/J to a better coach product but, again I highly doubt it. Yes, most PAX are flying coach/economy but, as others have said, airlines are trying to court full-fare F/J pax, not low-fare coach pax.
All of this says to me that the LH financial leaders are making a calculated investment, and nothing more at this point. Possible they might also view this as a hedge against any troubles with UA in future but, that is not as likely in my opinion.
Dan