Are marlin underlevers as sensitive to hold as springers? I have never shot one. And I would be willing to bet most UK shooters (whether you shoot rabbit, fox or deer) haven't either- and likely never will.
Agreed touching any barrel is a bad idea. I have never tested how much it affects accuracy at stalking ranges, however.
Yes airguns are cheap to feed. But the question is whether springers in particular are a good training tool. You can shoot PCPs, rimmies etc very cheaply too.
Indeed the springer lock time is awful. It's a real flaw. I don't see how the need to control this really helps people with modern shooting ?
Do those with a flinch really find shooting a springer helps ? If so- I would think other air rifles would also help at least as well- along with rimfires.
The vast majority of quarry shooting in the UK is performed off sticks or bipod. And you can't use either with a springer.
Giving a youngster with all the time in the world the lowest maintenance gun they can get- a springer- is a great idea IMO. But that's only because all they need is pellets.
But as an actual training tool for bigger guns- I don't think the skills the springer teaches (hold sensitivity) is useful as IMO it doesn't carry over well to 99% of UK shooting situations.
I'm not an especially good shot and have had very little in the way of shooting training. Just mucking about as a kid and carried on

I have heard for decades how good springers are for teaching- and I have always struggled with the thought process.
Any shooting is good of course and rhe more experience the better. But are springers patticularly good ? I'm not so sure.
Take 2 beginners- identical twins- 1 trains on a springer and one a PCP. Let's assume that the springer twin hadn't quit with frustration after wounding a lot more quarry.
A year later- they swap- sure the pcp twin will hate the springer. The springer twin will love the pcp.
Now who is the better shot with firearms ?