That seems like a bizarre claim... If anything the attacks, discrimination etc. would have encouraged assimilation. Voluntary conversion to Christianity wasn't particularly uncommon either (forced violent conversion also occurred). Even when national governments started expelling their entire Jewish populations staying and getting to keep all of your property ussually was an option. Yet most chose to leave rather than convert.
Yes, converted Jews might have faced discrimination and even violence (this depended a lot on the willingness to abandon your old customs and practices and varied hugely based on time and place, but seemingly became a bigger issue at the very end of the middle ages) but ussually they managed to more or less fully assimilate in a few generations.