Apple did not start anything.

PCs are the odd ones, all other 8 and 16 bit home computers were vertically integrated, most expansions were done via external buses connected into one of the sides, usually the back or right side.

With the race for thin margins at any cost, if anything thanks to Apple, is that OEMs realised going back to Spectrum, C64, Amiga, Atari ST kind of hardware designs payed off in their bank accounts.