I wonder how many of these might be avoided if a means of collection (e.g. a couple of TVMs) inside the gateline were provided, to which anyone arriving at the gateline without having collected could be pointed. While we know otherwise, the idea that you've made the journey so there's no need to pick the ticket up (or you might as well "go ter Laime Stsreets fer dat" as you're often asked to do for things) seems intuitive. Automatic refunding of uncollected walk-ups may happen, but nothing publicises that it will.
There's a TVM inside the gateline at MKC which is presumably there for that sort of purpose (it is still there, I looked on Friday). You can use it to buy a ticket from somewhere else (so it can be handy if wanting to go onwards to Bletchley, say) but I doubt that's its main usage. There's also one at Watford Junction, presumably for people arriving from the St Albans branch who haven't been able to collect yet.
At least Merseyrail does now do ToD, but the way it does it is subtly different from everyone else and as such it can catch people out, and there's no real need for it to do so.