My answers to your points:
1) It is most certainly not a heritage structure. It is not even listed as such. Why would a legal precedent for heritage demolition be set by the redevelopment of a non-heritage structure? It may be publicly seen that way by some, but it is not even remotely in the same ballpark (haha) as actual heritage structures that need protecting.
2) Size and capacity aren't everything. Sure, having a high seating capacity is great from a pure numbers perspective if you are only concerned about stats and rankings, but what good are 55k seats when you are only filling half on a regular basis? Keep in mind that the high capacity was for CFL football, it's more like 49k for baseball. Additionally, who's to say that they would be reducing it to 30k-35k seats? We are seeing reductions like that proposed for smaller-market teams like Oakland, but since Toronto is aiming larger, it's possible we will see something more like 40k-42k seats, which is more in line with what we see from larger market new parks like Globe Life Field (DFW) and Truist Park (Atlanta).
3) The City gets more tax revenue from office/residential towers, can levy more community benefit charges for this development, and can keep the Jays in the city. Blue Jays fans get a better ballpark/fan experience, in addition to potentially attracting more people to come to the games. From a development standpoint, land is used more efficiently, and there is never too much green space when it comes to downtown Toronto. Pedestrians in the area get a better-designed district than the windswept concrete plazas that the 80s had to offer. Keep in mind that the Skydome is also huge in comparison to other ballparks. It is grossly inefficient when it comes to space.