"Pushing" the maximum cog for a rear derailleur depends on the length and shape of the derailleur hanger. Some bikes will take larger cogs with a given road derailleur, others won't.
Here's an idea for a business--cast extra long derailleur hangers for the popular bikes so riders don't have to buy MTB units. Shifting sharpness would be lost, but at least the capacity would be there
When Campagnolo introduced 11 cogs, one wag wrote that he'd upgrade when they figured out how to make an 11.5t cog. Little did he realize that the best use of that extra cog would be to eliminate one more double-step at the slow end of the cassette, or, as Shimano is now doing, concatenate one more double- or triple-step down there.