Please spend an hour at St Pancras Low Level in the morning or evening peak, watch the peak direction trains from/to the ECML, and then you eill see how popular Thameslink is for ECML commuting.
I'm not up in time for the morning peak, but at least half of my day trips out finish on one of the evening peak Brighton-Cambridge trains, and I've boarded at every station between East Croydon and St Pancras. There is a huge amount of both alighting and boarding at all of them, and Finsbury Park too, with all sorts of people making all sorts of journeys. Even at East Croydon, the trains arrive quite full, they are packed by the time the train leaves Blackfriars, and they only start to feel empty after Hitchin. And because class 700s have loadings on the information screens, I can see how the train is filling up and emptying out.
I'm very cautious about interpreting the ODM data for the Thameslink Core stations. It is important to remember that the ODM matrix is based on ticketing data, and various tickets bought at (for example) Cambridge have to be distributed amongst Kings Cross, Liverpool Street and Thameslink core stations in the ODM matrix based on assumptions not data.
City Thameslink is the 16th most popular flow from Cambridge, Farringdon is 26th and Gatwick is 28th. Oxford is 39th and the highest without a direct train, followed by Edinburgh in 44th and York 48th.
Excluding stations in London and East of England, for Cambridge Gatwick Airport is first and Brighton second.
For me, the most relevant comparison with Cambridge-Brighton is Cambridge-Leicester. Brighton is further away from Cambridge than Leicester, Brighton is smaller than Leicester, and both have direct trains. Cambridge-Brighton is 26884 in the ODM, Leicester is 10294.
I'm also sure that someone going on holiday once a year, even a family with luggage for instance, is not going to be too bothered about having to use the Tube once a year.
I am sure that you are wrong here. Families with luggage are very bothered about using the Tube. And the same goes for elderly people and people with mobility issues. The alternative to the direct rail journey Cambridge-Gatwick isn't using the Tube, it is going by road. As it happens, one of my neighbours had family visiting from Spain earlier this month. They flew home from Gatwick and got the Thameslink from Cambridge. They would not have gone by rail without the direct train, a family member would have had to drive them to Gatwick in the car.
For both Gatwick and Brighton, the 2024/25 numbers are depressed by the large number of weekends with no trains because of engineering work.