It's still a lot faster than it is now. Currently from Malvern Town Centre the fastest way (to say Bloor/Yonge) is to take the 131, which takes over 30 minutes to get to to Kennedy, assuming it doesn't get stuck in traffic (at least accord to Google maps at 8 AM - personally I'd get off at Scarborough Town Centre and take the SRT ... but Google seems to think this takes longer by the time you catch the SRT).
Once the SRT is extended, it would be a 6-minutes route to the Sheppard/Markham SRT station, 17 minutes from that SRT station to Kennedy (assuming that the 3.5 km extension has the same average speed as the existing 6.5 km routes), and say 3 minutes to change. That's 26-minutes. And no traffic jams over the 401. And the time to change at at Kennedy will be significantly reduced with only one story to climb, instead of 3.
Less travel time and a more reliable service. How is this not a benefit to the people currently living in Malvern? And after 2020, Metrolinx still is planning on building the $386-million extension to Malvern Town Centre. They won't get that anytime soon if the subway is extended to Scarborough Centre.