It's true the jury is still out on this.
The Skywalk wasn't a frequet transit connection, but this will be -- it's Canada's busiest railway station that will become 2-3x busier in the coming years with all-day 2-way service coming to several routes, and 15-minute electrified service. Obviously, if that is rolled back by the next provincial government, I'll definitely have to err on the side of not-being-viable.
I think Union revitalization completion pretty much politically nudges even Conservatives (at least eventually) towards 15-minute electrified service into an almost certain eventuality on at least four GO routes (or two through-routes) -- whether 2024 or 2035. Otherwise, Union will be extremely overcapacity for quite a long time to come.
Even York concourse today, is fairly quiet offpeak, owing to current lack of offpeak train service. Now imagine that spread across Bay and York. That would even make the Union shopping a difficult sell without VIA HFR and 15-min GO RER, especially with missing York-side connections needed to help serve the massive downtown lunch market.
So, yes, I see the problem for York moat viability.
But, assuming 15-minute electrified service happens, and at least one York-side PATH connection complete to the rest of the PATH system, the York moat shops become viable, especially since the York moat will be the brightest indoors space that's out-of-the-way of the TTC madness (Bay moat) -- the bright dry indoors moat space is a draw itself as part of the Union mall. Lovely artwork, attractive decor (maybe Parisian cafe, complete with ironwork/iron fence, greenhouse-league high end planters, lovely smell of rolls, against a north wall -- well illuminated and "a reasonable pedestrian flow" ambience (not too heavy, not too quiet) -- BAM, the right mix -- yes, people will go to this cafe once in a while and relax for lunch under the skylight.
It may be no Galleria Vittorio Emanuele II (google it), but with a nice classic floor, a few planters, good climate control, and a good bakery aroma -- that would be enough to draw people here as the only super-skylighty place (sorta like a themed area) at Toronto Union, and a kind of a refuge from incredible TTC / Bay Concourse madness.
So, yes, it could go either way. A retail mess worse than Skywalk, or a resounding success.