Until the creation of TfL Rail saw local services being split off from Greater Anglia there was at least one Class 315 evening peak hour 'all stations' through train to Southend Vic.
I think that most Southend Vic passengers would complain if all trains became all stations - even if the overall journey times were faster most passengers would prefer trains that make fewer stops.
Half hourly off-peak and all day weekend (especially in the summer) might entice passengers from London (and even stations on the GWML) to go to the seaside by train, alternatively using Romford as an interchange location could also be acceptable.
The benefit of encouraging people to change at Romford to trains that miss out all stations between there and Shenfield and then call at all stations to Southend Vic is that there are fewer station stops. Going to Shenfield on 'all stations' trains is tedious.
Passengers would need reminding that Oyster PAYG is not accepted the whole way, although a better idea here would be a weekend and off-peak school holiday excursion paper ticket which perhaps also let passengers benefit from discount offers in Southend (even if only for a ride on the pier railway!!!)
Simon
ps: When I worked at Chelmsford (starting my journey at Ilford) I always changed trains at Romford - and not Shenfield.