Certainly there's been more growth around Bletchley, but the primary centre of white collar employment is CMK (and that isn't likely to change, though the number of days a week might), and blue collar employment is more likely to have commuters from the Marston Vale than Oxford and Winslow (because people don't commute long distances to warehouse jobs as the pay isn't enough to cover the season ticket cost). It's also the hub of the bus network, or what's left of it - all remaining timetabled local bus routes serve MKC, and only three or four of them Bletchley.
Bletchley is primarily residential and very much a traffic source rather than sink (except when the footy is on), and that won't change either nor is development planned that would make it change. It's also a poor Parkway because it isn't directly connected to the grid - this is quite visible from the fact that even though LM tried reducing the cost of parking at Bletchley to just £3 per day (MKC is about £8) they didn't draw the commuters in and ended up removing half the double-decking and shifting it to Leighton Buzzard (which, incidentally, may actually be a better Parkway to London from the likes of Newton Leys).
Careful
That population figure you quoted is Bedford Borough which includes all the way up to Sharnbrook.
Bedford itself has only 80,000 population, 106,000 if you include Kempston, Biddenham etc (but don't go there and tell people they live in Bedford!).
MK is already up to 315,000 according to some sources, I imagice that similarly presumably includes Olney etc.
So I was a bit premature saying MK was roughly the size of Sheffield, but it will be soon enough and it dosen't detract from the point that it really ought to be the core focus of EWR planning.
A quick Google of "Milton Keynes Expansion map" shows (from the MK Council website) a whole area around Ridgmont, plus the area roughly between Bow Brickhill and Aspley Guise. I.e. exactly where EWR will be going through the middle of. The centre of gravity of the whole borough will shift southwards slightly.
They are also planning and have started to festoon the Marston Vale with large numbers of houses (tens of thousands) but they are in Central Beds not MK. That is why the A421 expressway from M1 J13 to Bedford was built.
Hence closing places like Kempston Hardwick, located at Wixams, a new town under construction which will have a ~10,000+ population and Millbrook (located at Marston Moretaine), formerly a small village which has seen housing decelpment increase it to ~5,000 with further large developments under construction or planned....is not such good forward planning.
Ditto shutting Bow Brickhill given the housing development you mention in the area.