Why isn't the point you begin descent part of the flight plan?
Because the Top of Descent (TOD) is calculated in flight based on current weather data, STAR/Transition assigned, directs received during the cruise portion of the flight, vectors received taking you off your planned route, etc.. The FMC does the calculation for you with higher precision in flight.

For regular ops, ATC can use the rule of thumb 10.000ft / 30 NM to calculate an estimated TOD. In most cases, they will not need to do that however, as they can give instructions to aircraft to plan a certain STAR entry gate at an assigned flightlevel/altitude.

R: DLH123, when ready descend FL240 to be level\to reach at ARPEG

During lost comms, you are expected to maintain your filed or last assigned flightlevel/altitude until you reach the clearance limit. From there you will begin your lost comms procedures which usually include holding at the Initial Approach Fix (IAF), descending at the IAF to the approach altitude, and commending the standard approach procedure for the active runway.