Currently, a trip fro Union to other GTA stations like Burlington costs roughly $10 with Presto - do you think it is fair to other GO riders traveling the same route if only I get to benefit from reduced GO fares just because I rode the TTC earlier?
A further point to consider - the vast majority of TTC riders hardly ever touch the GO system as demonstrated by lack of familiarity bordering hostility towards Presto. Why should GO transit make extra fare concessions to a group that doesn't even use Presto and has vastly different travel patterns as its core customer base? If anything, Metrolinx should strive to include a reduced TTC transfer fare into the already high GO fares, not the other way round, which will be an immediate value add to the vast majority of current GO passengers.
You cite the combined cost of TTC+UPX from High Park to Union as $7.61. The question is, do you think it is worth to pay the extra $4 to get downtown in 15~20 minutes - 3 min to reach Dundas West, 5 min to walk to UPX platform, and 7 min to arrive at Union vs. the 30+ min on a subway? For you that may be a deal breaker, but for others $7.61 may well worth the money.