Unknown mechanics? My first suggestion would be to get a second opinion, but unfortunately, what they are saying may be true.

If the bearings are the cause, one diagnostic that could be done would be to install an oil pressure gauge in place of the oil pressure sending unit and watch for fluctuations in the rpm range you mentioned. Normally, the pressure will increase and be constant as rpm's increase. Bad bearings will cause a drop in pressure, or at least fluctuations.