I think you are being very over optimistic here. Usage will be higher than the Network Rail estimate, but they won't be out by such a large amount.
I look at demand from the Biomedical Campus, and it isn't growing a lot in the next 5 years. Current projects are:
- AstraZeneca (AZ) are nearing completion on the Rosalind Franklin building, but this is mainly for existing staff being relocated from offices near the main station. AZ have another building in planning but no commitment to build it yet.
- Discovery Drive will be built out by 2031 but mainly for GSK with employees relocating from Stevenage, hopefully some of these will use the train.
- The new Children's Hospital and Cancer Hospital do not have approved funding yet, the chances of either of these being completed by 2030/31 are diminishing.
Big numbers at Cambridge South are going to be at least partly people switching from the main station, not new traffic. I actually expect a dip in usage at the old station because of this (by way of comparison, I shall be looking out for how much Chelmsford falls in 2025/26 as a result of the opening of Beaulieu Park). Also, the only big development near the old station likely to complete before 2031 is Botanic Place.
At Cambridge North proposals to expand at the Business Park and the Science Park are still in early stages of consultation, much growth there before 2031 is unlikely.
In addition to employment, employees need houses to live in. Coming up between now and 2031 only Waterbeach is likely to deliver big numbers for the railway. Northstowe might generate some traffic between North and South, but that should be netted out for a Cambridge total.
I also look at supply. From the south side there is plenty of capacity, but it is difficult to exploit. From the north side, there is plenty of demand but capacity is constrained. Rail travel from/to the north can't expand that much without new capacity, and there isn't going to be much of that before 2031.
In the short/medium term, increasing capacity on the March/Peterborough and Newmarket/Bury St Edmunds services should be the priority. They are the routes that have capacity constraints for inbound commuting. What's needed is a timetable for Ely-Peterborough that prioritises Cambridge, and redoubling Cambridge-Newmarket, preferably with electrification and through trains to/from London. That would also help to accelerate the opening of Cambridge East.
Brighton is the most relevant here, because it illustrates the impact of geography. In the case of Brighton that's the South Downs and the sea. Brighton's high usage comes almost entirely from its one route through the South Downs to/from London (including Gatwick), and local traffic on the East and West Coastways. For Cambridge, the Fens are a geographical barrier too: there is only one rail route across the Fens and its capacity is limited.
What will prompt me to reassess my position is delivery of East West Rail, especially if there is a Bletchley North Curve too.