The 4 Coaches do fine until East Croydon/Purley (off peak) from where most of the passengers alight (ECR) . These 4 coaches then become 8 or 12 from the other service that conjoins at Redhill where the chance of getting a seat is highly likely (with the gained carriages) to ECR either from the Reigate Section or the Gatwick airport part. Sure the Thameslinks absence is irritating but able bodied people can stand for 20 minutes to ECR and change. Its one island platform over its not like you are walking from one end of Clapham to the other.
What Time was it? They occasionally put Southern Brighton bound into Redhill (Sundays Only) but very rarely Thameslink, let alone on Saturdays. Perhaps short notice maintenance on the quarry line?
Anyway It was probably needed. Redhill destinations southbound are really poor. Apart from Maybe Gatwick airport where one can change onto a Brighton service but it's only Thameslink and with the absence of the 2tph from Victoria, these trains stop everywhere and are often seatless by the time it arrives at Gatwick where then more people get on rather than off. I don't think Redhill has gotten a frequent Brighton service since the 2018 timetable change. I am sure Minstral25 lives close to Redhill basing off their amazing knowledge of the surrounding stations and services so maybe they can help me if I'm wrong!