B737 NG Captain reported encountering wake turbulence on takeoff roll from an A350 that was landing on the intersecting runway and had crossed over the top of the reporter's aircraft.

Narrative

We were given position and hold Runway 30; traffic landing 1C; an A-350. As soon as the A-350 crossed over the top of our crossing runway; we were given takeoff clearance… right into the wake of the A-350 that just crossed over the top of our runway approximately 2000 down our runway. We started our takeoff roll very slowly because of my concern of the crossing aircraft's wake turbulence. As we accelerated; our indicated airspeed went up to 80 kt. and then decreased 20 kt. as we went through the wake turbulence of the A-350… my exact concern and I was aware of this possibly happening. Our takeoff roll continued normally as our airspeed increased.My question is; why is there not any wake turbulence separation for aircraft taking off when a larger widebody aircraft crosses over the top of a departing aircraft. After all; the landing aircraft is still producing wake turbulence that is descending right into our takeoff runway and path.One would think that with the runway configuration in IAD; taking off 30 and landing 1C; and other similar airport runway configurations; that there would be some concern of wake turbulence from landing aircraft on 1C affecting the takeoff roll of aircraft departing on Runway 30 - a crossing runway. I would think that a very small aircraft like a Caravan departing on 30 Right after a heavy landing on 1C would encounter some wake turbulence from the heavy landing on 1C because the Caravan would take off quicker right into the crossing aircraft's wake turbulence.My suggestion would be to not allow aircraft to depart Runway 30 if a heavy is landing 1C without having some type of wake turbulence separation. I understand the runways do not technically cross" each other; but the flight paths cross each other."