There is a very good reason numbers change - it's called local authority; them's that pay the bills get to make the decisions. Local politicians would be rightfully affeared if they heard that the province wanted to 'coordinate' road numbering. Lately, they are up to their eyeballs in provincial dictats and 'efficiencies'. We often seem to want the government do more but insist on paying less. Although the more improved and inter-connecting municipal roads are county/regional, I would argue that "most" of our road network in southern Ontario is owned by lower tier municipalities and designated as either concession or sideroad numbers (warning - they repeat, repeatedly).
I would argue that most people don't even know that Airport Rd. has a number and that it changes. I was under the understanding that most travellers navigate these days by GPS and just follow voice commands or the line on the screen. The locals already know where they are and they usually don't care what the number is since most roads have either an unofficial or official name.
Edit - It is difficult to compare Ontario with other Canadian jurisdictions since local governance structures vary. For example, in the Maritimes, counties have no administrative governance; hence, no tax base, hence, no county roads (outside of a few regional entities). Their numbered routes are, in fact, provincial routes, owned and maintained by the province, much like non-municipal roads in northern Ontario.