Adafruit etc. compete for Maplin's core(ish) electronics-hobbyist business, but they largely don't sell hi-fi cables or PC components, for example, and they're not on the high street. Argos does sell a significant proportion of the stuff you can buy in Maplin—blank media, HDMI cables, electronic goods, even computers—for all it also sells many other things which don't overlap with Maplin's stock. Of course none of these rivals is in the same market niche as Maplin (or Radio Shack): if Maplin and Radio Shack were being beaten down by companies in the same niche then it's unlikely that anyone would be questioning whether that niche is still viable at all.

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.