Even if you incorrectly assume that everyone is going specifically to Union Station you'd still be better off dropping Richmond Hill and 401 East GO bus passengers at York Mills than at Oriole.
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Line 1 runs every 4 minutes, so including 0 to 4 minutes waiting and 2 minutes walking between the platorms, it would take 27 to 31 minutes to get to Union after being dropped off at York Mills.
Timed transfers from GO buses to GO trains are scheduled with a 15-minute transfer time to minimise the risk of missing a very infrequent train. So it would take 40 minutes to get to Union after being dropped off at Oriole.
Even though it would take 401 East and Richmond Hill buses 7 minutes longer to drive to York Mills than to Oriole, York Mills would still be 2 to 6 minutes faster for the passengers headed to Union, and much much faster for passengers headed anywhere else. Sure, some passengers might still choose to take the GO train for its superior comfort, but that small benefit definitely wouldn't justify the 5 minute-delay that serving Oriole would incur for 401 East GO bus customers.