I think people are asking the wrong questions. Instead of whether Brampton has the ridership to justify LRT, we should ask whether even Mississauga has the ridership to justify LRT.
Consider 19 Hurontario's 20k boardings per weekday. 20k is pathetic by TTC standards. There are TTC bus routes with over 40k. Why can't Hurontario be the same? Clearly, buses are able to easily handle the ridership of Hurontario. LRT is not needed on Hurontario any more than it is needed on Ossington Ave.
Highest ridership bus routes, 2010-2012
32 Eglinton West 48,684
35 Jane 45,699
36 Finch West 43,952
39 Finch West 41,434
29 Dufferin 39,721
54 Lawrence East 36,277
60 Steeles West 29,819
34 Eglinton East 29,501
53 Steeles East 28,050
85 Sheppard East 27,146
95 York Mills 27,709
96 Wilson 27,700
24 Victoria Park 24,731
41 Keele 24,592
7 Bathurst 24,262
102 Markham Rd 24,152
52 Lawrence West 23,036
63 Ossington 22,694
116 Morningside 22,285
Viva Blue 21,547
165 Weston Rd North 21,361
19 Hurontario 20,554
196 York University Rocket 20,199
http://dmg.utoronto.ca/pdf/tts/2011/validation2011.pdf
(Note, some TTC numbers are from 2010, in the midst of recession, so they should be even higher. 19 Hurontario's ridership is from 2012 so it is inflated here.)
If you are a Toronto resident, you should ask why the province is providing full funding for an LRT line in Mississauga when there are 20 TTC bus routes that have more ridership.
If you are a Mississauga, you should ask if you'd really rather have an LRT to Brampton instead of Toronto.