I think the railbuses left in late 59 or 60. Interestingly, reports say they boosted traffic so much they had to put on a relief steam worked push-pull on Satudays. They were replaced by 2 Car Derby lightweights - Cl 108? - and steam. IIRC, it was steam-worked at the end. But yes, of course loads were not normally heavy.
I'm not in disagreement with most of what you write. I don't think Bedford - Hitchen could have survived even it YorksRob had been Minister of Transport c 1961-65.
And yes, even anyone living in west Hitchen (or Stevenage, or area) seeking to go to Wellingborough in 1961 may have taken the bus to Luton, then train for better connections. (And at Luton, the bus station was conveniently sited for onward rail travel.)
And yes, apart from odd days when a scout troop* decided to travel that way, I suspect the number of people using Bedford as an interchange to/from the Midland from/to the Hitchen line (and ditto Hitchen with the GN) was probably never more than ten-twelve per day, if that. Not an economically significant number.
* Yes, I know there was military passenger traffic to/from Henlow, but I believe that was usually catered for by specials.
But you misunderstand my point, intended for the OP. There were closures all over the network which effectively destroyed hundreds, if not thousands, of similar journey possibilities by rail because the replacement bus services were not designed to link in to remaining rail stations or services.
And, unlike today, with no internet, trying to work out an intinery from (say) Stevenage to Kettering once the Hitchen branch went would have been a pain, phoning bus and railways for timetable information. Or finding it in the local library. Yes, perhaps only 5 passengers a week would want to undertake such a journey, and yes, for a time there were other alternatives, of sorts (even by rail, for a few years you could go to Sandy, change for Bedford St Johns and bus it to Midland Road, then continue by rail - but who, apart from the odd hard-core travel enthusiast, would do that?)
Added up, all over the country, this only excacerbated the trend towards the car.
And what do we have left running east-west between London and Leicester - 100 miles which once had a slew of possible cross-platform, east-west interchanges - Luton (via the adjacent Bute St), Bedford, Wellingboro, Kettering (only east - and very poor, agreed) and Mkt Harboro?
Nothing, except the Marston Vale rump.
EDIT - Apologies to the Doctor - many of the closures mentioned here were not due to Beeching, of course.