No, it won't be anywhere near full coming from Malvern because Malvern accounts for only a fraction of the total ridership.
4% use Ellesmere, 8% use Midland, 11% use McCowan, and 21% use Lawrence East...that's about 44% and we haven't even added STC yet. 63.5% use STC (note: totals reach 200% when you also include the 92% that also uses Kennedy...there's about 3000 trips a day made on the SRT that do not begin/end at Kennedy, and most of these 3000 are to/from STC). Subtracting those 3000 gets STC down to 57% of the total, in line with the 44% from the other stations (not rounding would have resulted in an almost perfect 100% combined figure).
So, 57%, or about 25,000 riders, use STC station to get to Kennedy every day. The 131/132/133/134 combined see a daily ridership of 26,500, but the 21/129/130/38, the other main routes feeding STC, see a daily ridership of 32,900. Right off the bat, we know that less than half the bus patrons filling the SRT are from where the extension would run. However, we need to lower the feeder buses' share even further because STC itself generates many trips. Add, say, 5000 trips from the mall/offices/condos and you get feeder bus ridership of 20,000, of which maybe 9000, per day, comes from Malvern. That's 20.5%, lower than Lawrence East. 20% of existing riders will supposedly benefit from the extension.
Extending *anything* beyond STC basically just replaces the Progress bus, which only has 8100 riders a day. Extending whatever is done with the SRT to Malvern punishes the rest of Scarborough...and there's higher ridership bases already existing on McCowan and Ellesmere.