I don't think the issue was ever the wheel size or that BS marketing pushed the 26 out.
More people especially in the developed World took to road bikes for competition, fitness and pleasure. Also for commuting. Road bikes came with 700 because they are faster. To get speed, we lost a bit of manoeuvrability and ended up with toe overlap on smaller frames. It made sense.
Tourers are still with 26 because it is tougher with shorter spokes, better to handle with loaded gear and technology did not force it to become lighter unlike road bikes with weight weenie demands. And more importantly it is the most readily available wheel and tyre size in the developing world.
If we had not switched to road bikes, we will still be with the 26 as the popular wheel size. Nothing ever was wrong with it in the first place.