Others would know but I thought the GEML was full LST-Shenfield and poss Ipswich-Haughley jcn, neither of which are affected by this.
North of Shenfield is a 2 track railway with 6tph off peak passenger, including the 2tph Norwich fasts, and lots of freight to/from Felixstowe. It operates at full capacity.
Chelmsford has overall passenger numbers about 80% of those of Cambridge. If there was a rail option it would be hugely popular.
Proportionally Chelmsford is much more dominated by London commuting than Cambridge, with 1.5 million one way journeys to Liverpool Street and another 0.6 million to Stratford. About 60% of all Chelmsford journeys are to/from Liverpool Street or Stratford.
Cambridge-Chelmsford would not be hugely popular. Cambridge-Ipswich and Chelmsford-Ipswich are both about 30-35k one way journeys, I doubt that Cambridge-Chelmsford would be much bigger than that.
I suppose, thinking of a future with P9 at Cambridge and the EWR quadding at Shepreth - as that seems far more likely. Hence the Cherry Hinton
Care needs to be taken not to allocate the EWR additional capacity at Cambridge multiple times. Cambridge doesn't need more than 2tph to/from Stansted, with 1tph each to Peterborough and Norwich, though it would be nice if the trains could be longer. Platform capacity at Stansted Airport is the key to that, not what happens at Cambridge.