Let's look at the current service levels.None are very big, so most can't really sustain the kind of bus service to Cambridge (say half-hourly from 6am to 11pm) that people would see as good enough for people not to use their cars.
St Ives every 10 mins day, half hourly evening
Huntingdon every 20 mins day, hourly evening
Haverhill, St Neots half hourly day, hourly evening
Newmarket half hourly day, none evening
Ely, Saffron Walden, Royston hourly day, none evening
The busway clearly makes a big difference. My hunch is that journey times, especially not stopping much in Cambridge, are an important factor. Nice buses matter too.
And from further north. Car to St Ives, Longstanton or Trumpington for the guided bus is also popular for long distance journeys to outpatients appointments at Addenbrookes.A surprising number of people drive from further west to the St Ives and Longstanton P&R sites
Security is a big issue in Cambridge, which is the bike theft capital of Britain. Cambridge North has a very big bike park that is only lightly used because it is notorious for theft.I don't doubt there's also the opposite of people cycling to the Busway and continuing by bus, but judging by the bike rack size and occupancy that doesn't seem very popular (unlike at railway stations going to London where you get that a lot).
In Cambridge the tension between retail and congestion has been rumbling on for more than 50 years. Its not new. I'm old enough to remember the construction of the Lion Yard and the Grafton Centre, with their big car parks but no roads for the traffic coming in and out of them. I'm old enough to remember the crazy 1980s schemes for subterranean car parks under the Market Square or Parkers Piece. You're right that the Newmarket Road retail park looked a lot better idea in the 1980s than it does now. A significant event then was the opening of Queensgate in Peterborough which was a competitor for the shoppers from all of Ely, St Ives, Huntingdon and St Neots. Retail is not so important now, though it is still a big factor in travel to and from the historic centre.When it was built - about 30 years ago I'd guess - the local authority would have wanted it there to promote people coming into Cambridge.
Now they've decided to declare "war on the motorist" that changes the dynamic and a developer with a bit of foresight should look for a site just outside the reach of Cambridge City Council, easily accessible by the M11, A10, A14 etc -
If that really was the case then the shops on the retail park would be unviable.I suspect Newmarket Rd would be even worse for traffic if the retail park was converted into housing.