I don't want to wade into the wider debate of system amalgamation, but on the topic of this being Line 10 and there possibly being a bus route number 10 that is confusable, I think that's basically a non-issue.
A train (be it heavy rail, light rail, streetcar, or subway) is easily distinguished from a bus (BRT, local, or long-distance) in speech and signage. Anyone giving directions is likely to say "take the 10 bus" or "take the line 10 train" as the case may be, and any printed matter or signage should make it clear whether any particular route is a train or a bus.
Personally, I like the historical wink to Highway 10. It doesn't need to be a the standard for all lines or routes, but it's a neat gesture where it works.