I see a combination of 3 factors that work against the idea of extending the streetcar up Bathurst:
1) Bathurst is narrow south of Wilson; a streetcar would have to run in mixed traffic. A long continuous streetcar route would be less reliable than the 511.
Actually, 511 isn't bad despite running in mixed traffic. The relatively short length, about 6 km each way, probably helps. But if the line gets longer, a delay in any spot will affect the whole line.
2) If a long line is not desired because of the reliability concerns, then the subway station at Bloor is a good location to terminate both the southern and the northern service.
3) Although Bathurst has decent ridership, it is well below other busiest bus corridors. Currently, Bathurst #7 is served with antics running on something 9 min headways. If more capacity is desired, it is far cheaper and less disruptive to improve the frequency to every 6 min or every 5 min.
There is a possibility of building actual LRT in the north section of Bathurst (from Wilson to Steeles and then to the Promenade Mall). It could have dedicated lanes for most of the length, possibly combined with a short underground section under 1.5 km of Wilson to feed into the Wilson subway station. Not a top priority, but something to keep in mind.
Not directing this specifically at you by any means, but I wonder if the real reason that pretty much no one here thinks that running a streetcar to St. Clair West is viable is simply because the service doesn't already exist.
If it did, I can't imagine forumers calling for its replacement by a bus.
Check out the Streetcar thread by Wolfewood here: https://urbantoronto.ca/forum/threads/ttc-streetcar-network.27667/
They post several reasonable ideas for expansion, and in the very next post, literally every single idea is shut down because, essentially, the next poster can't fathom anything beyond than the current network.