There will always be a 'need it immediately' market, as there will always be a 'I'm buying it here from a moral perspective', but I wouldn't want to build a business (let alone one the size of RadioShack) just on those target consumers.
Argos and the like sell toasters, watches, cheap jewellery and gardening tools, so they're not in the same market.
Farnell (and RadioSpares, Mouser, and Digikey) are primarily industry suppliers who sell components to the tiny hobbyist market as a sideline, but make most of their money elsewhere.
The real competitors now are companies like Adafruit (US) and Cool Components (UK) who caught the Maker wave and are riding it very successfully - possibly because they sell clever, useful things, and they treat customers like adult engineers, not gullible children.
Radio Shack was already a legacy company by the 1970s, staffed by the kind of marketing geniuses who come up with ploddingly dreadful brand names like 'Realistic' and 'Flavoradios.'
The UK equivalent would have been Amstrad back in the 70s and 80s - purveyors of cheaply made over-marketed consumer electronics to buyers who were clueless about better options.
Clearly, RadioShack doesn't have much of a niche any more. Buyers can get better products more cheaply elsewhere, and the hands-on electronics business has moved in a direction their management doesn't have the talent to emulate.
A few - Maplin and maybe Argos - have a super-fast same-day delivery option, which is an interesting development. I'm guessing we'll see more of that in the future.
The big question now is - what is mall/high street retail for? It used to be the only way to buy, but when online is cheaper the rationale for spending metric crap tons of money on store rentals in prime locations is getting less and less obvious.
It's not just electronics. All retail is suffering.
There may be a niche for speedy delivery, installation and set-up of consumer electronics and white goods, because no one does an all-in-one like that, and usually you have to wait too long, especially for big items.
But I wouldn't want to try it without crunching a full business plan.
The online maker-shops may even be aiding Maplin in some respects, as they've helped to create and make popular new product lines like Arduino shields and RasPis which Maplin has been able to bring to the high street.
So while Maplin does have issues to face, it appears to be profitable (it's even been expanding recently) while facing similar competitive pressures to today's Radio Shack. Thus if Radio Shack is bombing in roughly the same niche as Maplin, it's presumably down to inferior management (or maybe debt load) rather than the niche itself being unviable.