It sounds even easier than the
missing 100 meter (give or take a few hundred meters) at West Harbour, which is held up chiefly by CN and manpower issues.
This sounds quite sensible -- extension of all day service to Mount Joy seems to be a no-brainer given such a low-lying apple for Markham & Election 2018.
I suggest sending this suggestion to Metrolinx, tweeting to @GOTransit/@Metrolinx and/or emailing Anne-Marie Aikens. Get yourself heard.
They have already actioned on some of my suggestions (possibly as part of a herd of suggestions by general public, but I think occasionally only because of me).
-- Mainly a few low-cost things (like signage mistakes) within weeks of my reporting.
-- Even a few semi-costly things within months of my reporting -- they installed a 2nd electronic schedule sign at the Bay West Teamway
entrance from my suggestion (I can't be the only one complaining) because often 1 sign is the timetable and the 2nd sign is service announcements (e.g. Platforms being closed). When there used to be only one sign, the schedule was always annoyingly replaced by "platform closed or schedule changes" big-text messages. (And the 2-sign standard is what they now use anyway in York concorse, so this is compatible with their future).
Certainly, they don't seem to easily action on very expensive or politically tricky things, at least not very quickly, as it needs to get through a political phase, but those free-apples and low-cost low-lying apples do seem to get heard if enough people let them know. They may already know, but may need a nudge or two from three or four Joe Q. Public members to accelerate a low-lying apple down the chain, at least to internally run some cost analyses on. Don't have little hope when sending suggestions to Metrolinx/GO, they do flag suggestions down in a tracking system for future consideration at least.
Apart from now-sensitive logistics like modifying GO bus routes, insurance/union/labor concerns (e.g. Lunch break minimum requirements, better layover facilities at Unionville for train drivers, etc?), GO wicket booth staffing concerns, the only other reason I can fathom is that they are having some kind of a future service modification that doesn't permit long layovers at Unionville, and they don't want to "cost themselves into a corner"? Like the opposite direction needing to be time-shifted to shorten layovers. I wonder... But they still plan hourly service north of Unionville, and this is an easy incremental similar to Mt. Pleasant.