I think part of the problem with Maplin was that a lot of their stock was sourced in China, which in itself wasn't bad - buy it cheap and put a markup on it so that you make a profit.
The problem came with the likes of Ebay and Aliexpress, Banggood etc. People realised they could buy the same stuff that Maplin was selling but at a much cheaper price, albeit with a couple weeks waiting for delivery. For instance I wanted a mosfet a couple weeks back, for the price of just 1 in Maplin, I got a bag of 5 from Banggood, with a 3 week delivery time. OK so I end up with more than I need, but they get filed away for possible future use. Similarly a bag (10) of resistors from Maplin cost more than a bag of 200 from Aliexpress and that bag of 200 was subdivided into 20's so I had a range of resistances.
Certainly online shopping is the death of shops like Maplin, just the same way it has affected the model shops.