Three things I've noticed.
1) it's only 4tph in the peaks, so arguably the point about a lot of trains is less of an issue, as East Grinstead has an hourly service off peak - this means that sharing a heritage line could be possible just with breaks in service for those times. If we're talking peak hour capacity, then the loss of Gatwick services are potentially less of an issue.
2) 2tph from East Grinstead are Thameslink services, as such you could deliver from Eastbourne 1tph (existing) and 2tph (Thameslink) and still find a lot of people use the new services because it better connects to other areas in London
3) the journey time between East Croydon and East Grinstead can be 13 minutes shorter by missing out most stations, if you timetable the stopping service far enough ahead of the faster services you could gain a bit of that 30 minutes back
For example 5tph:
xx:00 fast service (Thameslink)
xx:05 fast service (Victoria)
xx:20 stopping service (Victoria)
xx:30 fast service (Thameslink)
xx:40 stopping service (Victoria)
The fast services could extend to Eastbourne, giving it 5tph by retaining 1tph via the existing route.
Yes the second train per hour to Victoria would be longer but at (say) 20 minutes longer it would still get you to London sooner than waiting on the next service so people would still use it. Whilst that would put more pressure on the retained service, some of that would be reduced by being able to get a Thameslink train which would reduce the journey time to get to (say) Blackfriars (or at least make it much more attractive as it's then a direct service - as you could just sit in a seat when you get in and then get off in London rather than squeezing onto a train in Croydon to stand into Blackfriars).
Those 4 trains north of East Grinsted would be well loaded, but empty between East Grinstead and Haywards Heath and half empty towards Lewes.
There aren't enough Class 700s to send to Eastbourne.
Timetabling-wise, anything other than a perfect 30-minute cycle is a recipe for disaster.
Today, Eastbourne and Lewes have a clockface 30-minute service to Victoria and it's a 3-minute cross-platform interchange onto a Thameslink service at Haywards Heath. Nothing proposed by anyone demonstrates an improvement on this timetable, and nobody has demonstrated tangible benefits to other network users to make Eastbourne and Lewes users' sacrifices worthwhile.
A two-track railway south of Balcombe Junction handling a mix of stopping patterns is a challenge but hardly insurmountable. Capacity crunch is Haywards Heath northwards, but that can addressed by being able run Three Bridges starters, Gatwick starters (GatEx back up to 4tph with sensible fares), 4tph from East Grinsted for more hours, and separating the Cats and Tats. Those trains don't all have to start from Brighton. The infrastructure constraint is Croydon Area - once extra paths are released by a fundable version of CARS those paths don't need to take the scenenic route via East Grinsted all the way to Eastbourne.
The best case of a 15-minute journey time penalty (fast north of East Grinsted if you could get the timetable to work) would require Bluebell to be upgraded to 75mph - that means complete rebuild (ballast level) and resignalling. Network Rail would have to regain stewardship of the line and Bluebell would have to to pay access charges and have modern train protection equipment installed on their trains. It's a complete non-starter.