A lot of that is simply due to general drop in people taking up electronics as a hobby any more. Now if it can't be built around some uC unit and have every issue of the circuit or devices electronics operations handled in code its isn't worth playing with.It's not much related to the disappearance of all the other electronic manufacturers I mentioned.
Then add in that for those of us who still do the DIY circuitry online storefronts and wholesale supply companies from everywhere in the world give us unlimited access to their stock inventories of literally millions of components of every imaginable application and in any quantity anyone could ever use there's no way any brick and mortar store chain can compete.