Outside of cities, journeys are too long for it to be preferable for most people, even with electric cycles.
Not sure I completely agree that it's the distance that's the key, for me it's much more the characteristics of the route. My parents live in a “semi-rural” (I think that's what upmystreet used to say when it still existed and had ACORN classifications available!) village, and it's a 6km (20 minute) bike ride from there to the railway station in the nearby town. That's really not very far on a bike, and I've done it many times. It should be well within the capabilities (and desires, since avoiding congestion, parking time and cost, etc., in the town, all make any alternative which avoids those aspects attractive!) of the vast majority of the population.
The problem is that about half the route (maybe a bit less) is on unlit rural roads with the national speed limit. This makes it feel pretty unpleasant for cycling, especially in the dark or in bad weather. Even at the best of times, cars, vans, and lorries, belt along relatively narrow “old fashioned” roads (at high speeds, often ignoring even the 60/50mph speed limit), which don't have enough width to overtake a cyclist while remaining on the left side of the centre line, and so things generally feel pretty precarious at times. Better cycling provision (protection) would make it much more likely that more people would use the bike over the car (or the limited bus service).
Having said that, of course the number of people that would benefit remains very small, and so it's clear that when there isn't unlimited money, that money should be spent instead on infrastructure upgrades where the highest numbers of people will benefit for the highest number of journeys … and that's clearly, for now, in the larger towns, and cities, for people travelling to and from work and daily life (shopping etc.).
It's going to be really interesting to see how things pan out with the major (I hope!) upgrade to the Botley Road in Oxford —
https://www.oxfordshire.gov.uk/resi...future-transport-projects/botley-road-phase-1 — “Work to provide major bus, cycle and pedestrian improvements along Botley Road.” (whole article too long to quote here initially but I'll add it if someone really needs it!). What's encouraging is that “The study confirmed the value of a high-quality route that prioritised sustainable transport modes. Alternative options were considered but rejected because they either did not deliver strategy objectives or required significant works outside the highway boundary.”, which sounds very pragmatic and forward looking.
To get back to the core question of this thread, the cost-benefit analyses clearly haven't been done yet, but it'll be interesting to see how they look w.r.t. these proposals, how the ROIs of improvements which are targetted at pedestrians / cyclists / buses compare, and so on. This is on a single-digit kilometre route, which has heavy traffic consisting of private, commercial, and public, motorised transport, as well as human-powered vehicles and pedestrians.