There are no fix-all solutions for transit in Scarborough. Going all in on the LRT or all in on the subway in order to fix Scarborough's transit crisis will lead to equally bad outcomes. This one line/extension cannot do everything for everyone.
The LRT would've been much better at addressing local transport needs with Scarbrough, but it does nothing to address regional travel (including towards Downtown). That requires an additional solution (on top of the LRT plan).
The subway is marginally better at addressing regional transport, but its still a very poor solution. Of course people will advocate for it, because subways are all we know in Toronto.