I expect they're keeping the line names, at least for now, because that's how the vast majority of people know and refer to the lines. I fail to see how this is "holding them back". There's nothing inherently better about line numbers. Some cities use names, some use letters/numbers, some use colours. Is the TTC bus system being "held back" because people still talk about the Wellesley bus rather than Bus 94?
Having line names is useful in Toronto's network because there's a more-or-less 1:1 correspondence between streets and lines. In a city with a spaghetti network like Paris, you need an abstract naming system because there's no other option. But when you have the option of naming lines concretely after the street they run on, as in Toronto, it seems to me that that's a benefit, not a flaw.