Agreed. However, there is an obvious case for implementation of a TTC zoned-fare (which is what GO uses) across certain points on the TTC network at Steeles on the Spadina line and Yonge line if that extension gets funded.
Interestingly, once Presto is setup and TTC requires taps at all transfers, GO can offer a co-fare for TTC local bus service only (not subway, LRT, or express bus services). This would match pretty well with the 905 offering under the same logic (reduced parking requirements for inbound trips) without giving away the cake at Union.