Looking at the walk via Phoenix Road on Googleearth, it looks quite pleasant. The signs may be rather 'architectural' but they are present. Rather less clear at the Euston end but this could be worked up as Euston develops. A shuttle bus/ pods could develop (there is width) as demand develops and at relatively low cost (liitle infrastucture development necessary).
Heathrow was without link to Central London (for which there must have been huge potential demand) other than the 'Airline bus' to an 'air terminal' at Kensington or Victoria for YEARS!!
A Euston eastern exit/ entrance in the vicinity of Phoenix Road should easily demonstrate/ facilitate/ 'prove' what value there may be in further investment, esp if financial support is a bit short at the time.
'Passengers make their own way between stations'; I imagine that 'gatelines' and ticketing can be adapted to whatever seems most suitable come the time. Umbrellas, bus, tube, taxis also possibilities
"Less than half a mile"? Or a little more than that? And what's wrong with creating some small form of powered transport that potentially benefits a lot of people, whilst also encouraging them out of cars and aeroplanes?
If you want to express concern about people who walk around half a mile, try looking at just about every school in the country and see how much fuel is wasted transporting children shorter distances than this, whilst simultaneously depriving them of valuable exercise, and also preventing them from learning road sense. And do you really think children travelling to school with perhaps a small bag, need transport in comparison with rail passengers carrying suitcases?
I believe that hedgehog 'runners' are evolving at the expense of 'rollers'; similarly children without legs and with longer thumbs ... and as for being let out unsupervised?