Just looking at the network around me now, and where I grew up... the line to where I am now was completely pointless by the time of the bus, three miles long & served a junction in the middle of nowhere. The branches to the towns either side, well one was awkward to operate and the other ended in a badly placed station but that one might have been a handy place to terminate a commuter service now ( although looking at the demographics of that place I am not sure there's that many commuters ). So that's one definitely right, one very likely right, and one probably right, out of three.
The Exe valley line where I grew up is a slightly different story - north of Tiverton was an absolute basket case, there's one small town up there, a closed quarry, and then the Taunton-Barnstaple line which was pretty awful all by itself. *South* of Tiverton to Exeter though - 2-3 double deck busses running to replace the rush-hour trains? admittedly the bus in the middle of the day was usually quite empty by the time I was using it ( 20+ years later ), but by then most people had given up on the bus as it went round all the villages on the way & then got stuck in traffic. And the legendary traffic jams at Exeter from the north, still awful now. I'm reasonably sure the line was looked at as a whole, rather than the two segments it was built as, or the bottom half might have survived ( and add the line to Tiverton Jct - basket case as standalone & you get a loop of possibilities - that bit was massively overbuilt ).
Other lines - Barnstaple to Halwill via Torrington, certainly a basket case for passengers from Torrington south and I'll accept the whole line - however should have qualified for a grant to upgrade the clay handling facilities in the early 80s to keep the road traffic off the insane roads up there, but we didn't really do that until a few years later ( and anyway that's out of scope because full closure wasn't a 60s decision ). Okehampton-Padstow? shouldn't have been built in the first place. Okehampton-Holsworthy-Bude? these days that might get a social needs pass but I'm not really convinced ( pity, bet that was a lovely journey ). Ilfracombe came up recently & that one is mostly a victim of unfortunate circumstance, if it didn't have all those crossings it might be the end of the Tarka line now. At the time it wasn't renumerative & no real desire to make a good enough social case ( actually that one wasn't 60s either but near enough ).
So from those, four and a half definites, one probable but would fill a transport hole now, one unfortunate, half possibly a bad decision. Some of the closures might seem like they'd have been useful now, but really probably wouldn't have - a line from Taunton to all the holiday areas in North Devon? sounds a great shortcut, right? except the line was done on the cheap & wasn't very good.
Doing pretty well on the good decisions! the one's I've queried are either dormitary towns with connection problems, or remote towns with awkward connections - cars are just far too useful in rural areas, the only time they really aren't the best way to get around is either if you can't afford to fuel them, or you're going to a built-up area with traffic problems. Rural buses aren't really that attractive these days ( probably for a lot of reasons trains aren't - you have to get to the bus first, and then a rural bus is definitely not as comfortable as a train and still share traffic problems, so that's even less a reason to get out of a car ) but the 60s were a different era.
And, looking locally at the eastern side of Devon, Exeter didn't turn into the place the entire half of the county wanted to commute to ( as opposed to just go shopping in ) until probably the 90s - that's not something anyone would have predicted in the 60s. Taking a trip on the Barnstaple line in the 80s you could probably have struggled to meet anyone sometimes.
Taken holistically, "Public service" profits everyone including the economy. As you say though that should subsidise the service rather than the means. Given the trouble local authorities are having keeping a lot of bus routes running, this seems a literally foreign concept these days.
I think you might want to compare the SR line to Plymouth with the GWR one for gradients

the fact that the GWR line was already the main route even before nationalisation killed the SR one - much the same with the others you mentioned, really? mostly duplicate links and less useful than the route they're duplicating.
That is a point about buses ( one I brought up too ) - some places buses really would have been better than the railway, but I wonder how many people just went straight to the car.