My question is: What is the "cost" of wingsweep?
Reduced CL?
Off the top of my head there are several:
Reduced CLmax (proportional to cos(sweep) roughly)
Worse induced drag factor, at low M anyway
Higher wing weight (increased structural 'span')
Increased rolling moment/sideslip sensitivity (Dutch Roll, bigger roll response to sidegust, crosswind landing)
OTOH:
Reduced lift curve slope (less 'g' for a given up gust at a given wing loading/airspeed)
There may be others.
Going back to the original question, the usual explanation is that with a swept wing you can resolve the oncoming airflow into two components: along the span and normal to the LE (or quarter chord). The first has not much effect on Mcrit, but the component normal to c/4 is reduced by cos (sweep) so the airfoil 'sees' a lower Mach Number than freestream.
Cheers
CliveL