Both of these answers are valid.....if we presume people only go to Pearson airport from Toronto.
Creating a connection/extension of GO trains from Malton to the airport terminals means that the KW line has the ability to deliver people to the airport even if, god forbid, they live west of the 416 and dare to want to fly somewhere
Playing the devil's advocate for a moment, if we go with that POV of Metrolinx wanting people to use UPX as the fares are higher, people living on the Kitchener line west of Toronto can take the train to Weston and transfer to the UPX. Admittedly that takes a bit longer--while the transfer time to the bus at Malton and the drive is probably comparable to transferring to UPX at Weston and that trip, the train ride from Malton to Weston adds what, 10-12 min?--but I even recall someone in this thread who lives in Kitchener who did exactly this recently. Very doable, and if we're being cynical, it not only nets Metrolinx a slightly higher GO fare plus the UPX fare but it means they don't have to expend a significant amount of resources running that bus service.
Also, for the passenger, it's a bit longer and costs a bit more, but (not to rehash the beaten-to-death UPX fares argument) how often does the average GTA resident fly out of Pearson anyways, and if they're spending so much on a flight is at most $15 extra (1-stop UPX Presto fare, and 2 stops farther on GO from Malton) and a few minutes really going to be a dealbreaker? And while you could transfer to a bus, although it's a short trip the UPX is much more comfortable and significantly easier to take luggage on and off of. Also, for somebody who absolutely, positively wants to connect at Malton instead of going to Weston and taking the UPX, they could always take a cab from Malton--probably a pretty cheap and short trip, especially via UberX--or take a MiWay/TTC bus.
On that note, I'm not sure what the exact current services are, but if I recall correctly from debating doing it recently, taking TTC/MiWay from Malton to Pearson involves a fair bit of a walk from the station proper, at least at some times of day or week and if you want to avoid a circuitous route/transferring buses. I certainly think that AD2W on Kitchener justifies MiWay rethinking service to Malton station, i.e. having buses go right into the station proper and being reasonably frequent to the point that at least they connect to departing+arriving trains and wait for the connection. I think improved MiWay service would make more sense than operating an entire GO Bus route for such a simple trip--I hope that the TTC, MiWay, and Brampton Transit generally give serious consideration to better bus service to Kitchener Line stations in general once AD2W happens, not just Malton.
We'll see what happens. Presumably the GO bus link, if it exists, will be announced along with AD2W soon; if it isn't, then it's probably not in the cards. Hoping MiWay/etc do revamp their services though.