Not sure of an unlimited runway length, but I certainly know of a situation where the runway is short and higher MTOW can be achieved with a derate.
Correct, the reduction in the Vmca sometimes generates a takeoff performance benefit (higher MTOW) off a short runway.
V1 is the maximum speed at which it is still possible to reject the takeoff and stop the aircraft within the runway limits with V1 > Vmcg, and with the ASDA being often the most constraining limitation on a short runway, lower Vmcg, one can lower V1.
A lower Vmcg will lower the ASDR (lower V1) for a given takeoff mass, and lead to better takeoff performance when the MTOW without derate is ASDA/Vmcg limited.