That's a good question.
I would say that any town with 30k plus population "needs" a railway station, which includes Wisbech. That isn't a minimum either.
But that's a daft arbitrary measure - take the example of Dunstable, cited elsewhere.
Follow your logic and the line should have been reinstated - but it would have been a single line (which the branch was in any case) with an at best 30 min service to Luton and London. You can't go the other way out of Dunstable because the formation is long gone.
Yet, Dunstable has Leagrave station 3 miles from its town centre - so less for anything east of the town centre which is most of Dunstable - which already gets 4 tph on Thameslink.
Whereas the busway has every 10 minutes which can serve Luton town centre and the airport going east and Leighton Buzzard, MK and other places going west.
The other towns which have a population of over 30,000 and no station are:
- Gosport - unlikely to ever happen, has ferry connection to Portsmouth which gives quicker journey time to London and beyond. Busway using rail formation to Fareham.
- Dudley - Tipton and Dudley port are close by. More likely to be reconnected with extension to Midland Metro.
- Newcastle Under Lyme - Part of Stoke-on-Trent conurbation. No chance of reinstatement as formation of old NSR long gone.
- Washington - May happen if Leamside line reinstated.
- Waterlooville (never been rail connected) - Rowlands Castle, Havant and Cosham all close by.
- Halesowen - Old Hill close by, the line which served it was minor and station closed in 1958.
- Leigh - now has busway.
- West Bridgford (never been rail connected) - Nottingham station less than 2 miles away.
- Kingswood (never been rail connected)
- Skelmersdale - being looked at by Merseyrail
- Canvey (never been rail connected) - Benfleet station covers the area.
- Blyth - already being looked at.
- Woodley (never been rail connected) - Earley and Winnersh Triangle close by.
- Coalville - Depends if Leicester - Burton ever happens.
- Abingdon - was end of a branch from GW Oxford line. Both Radley and Culham are about 3 miles from centre of Abingdon.
- Swadlincote - line long gone - station closed in 1947.
- Wisbech - being discussed.
- Rushden - was on a minor branch from Wellingborough to Higham Ferrers. Formation lost, wouldn't be viable in any case.
So of the 18 which meet your arbitrary population criteria, 5 were never rail connected.
Of the remaining 13:
3 are already being seriously looked at - Skelmersdale, Blyth and Wisbech.
2 have busway connections in place - Leigh and Gosport
2 have rail connections close by - Halesowen and Abingdon so are unlikely to ever justify a reinstatement.
1 will gain connection by light rail in due course - Dudley
2 may happen if other schemes go ahead - Washington and Coalville
Which leaves 3 which definitely won't happen - Newcastle Under Lyme, because the formation has gone and nobody is even vaguely suggesting re-building the NSR network around the Potteries, which is what would be needed. Swadlincote - same reason, would need a new line, more likely to get 'parkway' type station if Leicester - Burton happens. Rushden, formation lost but it was part of a minor branch from Wellingborough to Higham Ferrers. More likely to get a nearer station if an intermediate stop on the MML is put in between Wellingborough and Bedford.