Realistically, any reasonably responsible businesses owner will always be thinking about their assets with these lenses.
It's easy to spitball ideas when you don't have to foot the bill, and that's pretty much what you are doing. If they were to follow your proposal, these guys would have anywhere between 50 and 200 million dollars shortfall to make up for, between lost revenue and maintenance costs, and that is IF the market conditions allow for the development to be profitable and start recouping the costs immediately after construction is finished.
LaSalle and their investors are businesses, and as much as they might want to engage in the betterment of the city, their main goal will always be to make the most money possible out of the assets they own. Within these parameters, their best play would be to demolish and redevelop the HBC space, as it was already pointed out. It would stop the bleeding on the current maintenance costs, which could itself finance the redevelopment, and become a profitable asset much sooner.
But again, if you are confident that you have the best solution, I suggest you approach LaSalle and bring in the $200m financing to back it up, assuming all of the risks. I'm sure they'd be thrilled to follow your vision.
But then again, last I heard, your capital raising efforts were pretty much unsuccessful, do there's that.