WH Smith High Street has been run on a limited budget in so many locations for literal decades. My nearest store still has the same carpet and old shelving units that have nicely yellowed with age. It never got the facelift some stores got, and has no post office. At times it has zero staff on the floor and a button to call for assistance. Despite having no staff on the shop floor, I doubt anyone even bothers to go in to shoplift as it's so dead!
But WH Smith Travel is a totally different ballgame, and has such high profit margins and a captive market that I expect it could survive indefinitely. And I assume there's WH Smith News that distributes papers and magazines, and takes a pretty large cut of the cover price. Presumably like the supermarkets, at least when they had a large number of magazines and papers, they also charge for placement in a good position, or having shelf edge promotions etc.
I really can't see how high street shops can survive, but they've lasted waaaaaay longer than I ever thought so who knows?