Journey time on Thameslink services between London Bridge and Horsham is 1 hour 10 mins (Monday to Friday). On Sunday, the Thameslink services between London Bridge and Horsham also calls at Norwood Junction, Purley, Earlswood, Salfords yet journey time is 1 hour 11 mins.
the service would be 12 cars, so I don’t get why they would be standing especially off peak?
That 1 hour 10 mins already includes stops at Merstham, Coulsdon South, Redhill and Horley. However, that time already includes timing breaks to allow extra calls at Earlswood and Salfords when the 9R (Three Bridges - Bedford) services are withdrawn.
In any case you should look at the East Croydon to Horsham times as the London Bridge times are not representative - 1C's originally from Victoria take 34 minutes (calling at GTW/TBD & CRW only, whereas the Thameslink takes 53 minutes calling at MHM/CDS/RDH/HOR/GTW/TBD/CRW/IFI/LVN on the way.
So, extending Thameslink to Bognor means adding 19 minutes to journey times for the people of Bognor.
Also, prior to 2015 the 12 coach trains that used to ply the "via Redhill" from Arun Valley were always packed on arrival at Redhill with all seats taken - COVID or not I would expect the trains to be equally full and no seats available from Redhill after the change.
They could do that by merging services.
For example, the Bedford to Brighton service could call at Norwood Junction, Redhill, Earlswood, Salfords and Horley.
The Bedford to Three Bridges services could run between Bedford and East Croydon calling at stations via Forest Hill. (I know many stations have short platforms but I’m sure that won’t be a problem unless you swap 8 cars.
Gatwick Airport would still have 6 Thameslink services per hour with also many other southern services to run to London Victoria.
There is a reason the routes are as they are - there was a massive debate up to the 2018 change about the best service organisation. The current timetable keeps routes from getting in the way with each other - your suggestions whilst valid ideas would probably cause more disruption, which is why those decisions were not taken in 2018.
For example, Redhill not having a Brighton route service keeps the Brighton train away from crossing from slow to fast lines south of Gatwick. Which is also why the Horsham train comes through Redhill as it is then already on the slow lines which is the same side as the Horsham route split.