Some of the older highways definitely didn't make sense by the 1990s. There wasn't really a need to keep 500-series secondary highways in Renfrew, Lanark, and Peterborough Counties on the books where there was a county government able to take them on. Short intramunicipal connectors like Highway 88 in Bradford weren't really necessary either. Same with Highway 11 south of Barrie, or Highway 93 south of Highway 400.
But Highway 9 was the opposite extreme, and the worst of a long list of highways (I'd add other long rural highways, like 86 or the south part of Highway 28) that never should have been downloaded.
I agree with the 500-series highways, but I don't know about highways like 11 and 8. If someone wants to take a scenic route, it's nice to have consistent signage along the route.
There's a reason why the US Highway system has remained largely intact despite many of the routes being supplanted in importance by an adjacent Interstate. If you want to drive around a town, take the interstate. If you want to drive through the town, take the US Highway.
Largely because when you turn ownership over to somebody else, they get to call it whatever they like. It's not just maintenance. Some jurisdictions did manage to keep the county designation consistent with the previous highway, some didn't even try and some couldn't because the number was taken. Simcoe couldn't designate former Highway 11 as County Rd. 11 because the number was already in use (Old Barrie Rd.) but they did manage to keep several consistent.
But my point is I don't know why maintaining the current number and signage wasn't made a condition of the download. Municipalities are creatures of the Province, so I would think the Province could have mandated that signage remain consistent (including continuing to use the King's Highway shields). Just look at the US. The states own the vast majority of the Interstate and US Highway networks, but AASHTO controls the numbering and signage.