I agree that I wouldn't be seeking to carry large items bought from a shop on a train, although, that said, I did buy an iMac 27-inch which I took by taxi from the Apple Store in Regent Street to King's Cross and then on the train.
That was an exception to the rule, and I'd more likely drive to a retail park than go shopping by train if I intended to buy lots of things.
I now consider it a lot easier to buy online and, especially as I now work from home, get it hand delivered to my front door the next day instead. Works for me and, by the sounds of it, a lot of other people.
Amazon can now deliver to a number of locations for collection at your own convenience now, which solves the issue of waiting in all day for a courier to turn up.
Looking around where I live and a number of different town centres, the council decision to pedestrianise many places effectively killed them off many years ago (as in the late 1980s for places like Hoddesdon, Herts and Waltham Cross). It's been a slippery slope since then, and with both the anti-car policies and now increased competition, most high streets are doomed regardless of what the council might try and do now. It's too late.
There's clearly not going to be the money to re-open streets to cars, drop rents and modernise crumbling shopping centres that would need millions to modernise to even get close to modern shopping mall standards - and any half arsed approach will just be more money down the drain. Sorry if that sounds defeatist, but I can't see what will save the high street besides getting in shops that people want to shop in.. and that's most likely stuff like Costa/Starbucks/Nero and Primark.