The rot set in much earlier than the onset of online trading
Firstly they all became carbon copies of each other as chain stores trashed wages, cosied up to shopping centre developers and quietly moved all their product manufacturing to far eastern sweatshops.
Then the taxman (that is all of us) was thoroughly shafted again when Boots et. al. were bought out by debt laden private equity swindlers (namely the Banks, the big accountancy firms and other less savory lenders) and taxes dried up against these, lets face it, imaginary debts.
Finally, with the lie of 'job creation', every town with a population of over 100,000 constructed a massive, unnecessary, bland, boring, sterile, daylight free, retail PRISON disguised as a 'marketplace' that ensured local businesses could never survive or negotiate downwards the hefty rental prices demanded by the shadowy world of the 'Shopping Centre and Multi-storey Carpark Mafia'. Everything else left half standing on the periphery was mopped up by the out of town Retail Park for those that reflexively gag each time they approach the doors of a shopping centre and needed an alternative to purchase their essentials.
Any vestige of grass or weeds are usually destroyed the moment they pop up through the hard landscaping of gum spattered, shit-brown cobble locking. Trees, if any, are never allowed to age. The shopping centre at Salford Keys is a good example of how these developments set out to destroy any pleasure in shopping by turning it's back to the canal basin. You could spend three hours there and be completely oblivious to the waterway.
Anyone been to Eastern Europe recently? Town Centre's built in mind for people that might actually have something to say to each other on a Saturday afternoon. With proper, spacious, landscaped parks and pedestrian areas in the centre where you can spend a really pleasant few hours; whether spending or not. And a mix of retail that doesn't set out to shaft local traders.
My proposal, for a grim town like Blackpool for example, would be to knock down a block, or even a few blocks of empty retail shops before it's too late. Plant a nice park, and start with adding a children's playground. I'd also open the city gallery on a Sunday.