To the folks surprised there isn't more high end retail in the core ... why is that such a big surprise ? I won't comment on how much retail there, rather the "increase in it". Now even with all the people moving into the core I'd argue very few of these are at the very high end of the spectrum. Lots of middle and upper middle class, folks that couldn't afford to step foot in many of these stores. You may argue, what about the luxury condos ? I'd argue many of those folks were already in the core or close to it and already shopping in the vicinity.
Also keep in mind there has been a lot of new non-high end retail throughout the core, namely due to retail space in condo developments.
I doubt very much that retailers in Yorkville are trying to cater to the influx of 30 somethings moving into the core and living in 400 sq. ft. condos, so I would agree with you on that point. Bloor/Yorkville's appeal is to the small percentage of people who buy thousands of dollars of stuff at a time, whether they live in the core or not, plus tourists who will shop at these places when they visit Toronto. As has been pointed out many times in these threads, malls have one property manager which can consolidate its efforts into expansions, lease negotiations, and long term strategy. The BIA's don't have this so it takes longer to close deals in neighbourhoods like Yorkville. Retailers may not want to wait that long if another option exists to get into the market.