"From a gross profit perspective, Maplin was incredibly profitable (the full accounts made up to 28 December 1996 show gross profit of £15.6m on turnover of £32.6m), a result, perhaps, of its broad appeal to a mix of different clients "
I'd say the reason for the profitability was the ludicrous prices charged in Maplin stores. And there lay the roots of the demise. It wasn't "online" that killed Maplin it was "competition". Maplin had originally, in the catalogue days, been both competitive and extremely helpful. The catalogue was a brilliant source of information and something I looked forward to receiving each year. The many examples, plans and technical info sections in the catalogue encouraged experimenting and that lead to buying components, cases, etc from Maplin. The shops were originally the same, staffed by people with an interest and willing to help. The shops were also well stocked.
The rot set in partway through the 90s. The knowledgeable staff started to drift away, the availability of stock became intermittent. By the 00s that had turned into guaranteed unavailability of almost everything. I recall wanting some aluminium knobs only to be told that they weren't a stock item and the wait for delivery was two weeks. I could buy them online for a tenth of the price and have them delivered next day.
The suits were more interested in pushing very expensive tat and gouging on the price of cables and cards. I suspect that many customers stayed on though inertia but eventually everyone gave in to the fact that you can buy the leads at a fraction of the price in a supermarket / DIY store and any "unusual" components like knobs, resistors, cases, PCBs, etch baths etc. could all be obtained faster and cheaper via eBay/Amazon.