I am aware of the history regarding the alternative proposed routes and the involvement of the NIMBY's along Main, but a rejection by a bunch of NIMBY's should not be grounds to automatically gravitate towards a significantly more expensive option which does not align with the density of the corridor. Various different alignment options could have provided different costs/benefits in terms of connectivity (e.g. Main is more direct, Kennedy + Rail Corridor would have allowed access to William Osler and Brampton GO), but the decision to construct a tunnel is different from the choice of alignment in that it is a purely political decision made in spite of the reality of the situation. A lot of well-earned disdain has been directed to the parties involved in the initial shortening of the LRT in Brampton, but the reality is that by digging this tunnel, not only are we giving them what they wanted, we are also showing the rest of the province what they need to do to have their way.
I agree with you that connecting the LRT to Downtown Brampton is a great idea with network connectivity benefits beyond the weak density of the place, with that said, the network impacts and the bus relief which you mention are brought about by the LRT itself, not the tunnel. There were far far cheaper ways to introduce those benefits into the network, but the province has bent the knees and chosen the same transit for a far higher cost. Remember every dollar spent on this, is a dollar not spent on growing bus networks, constructing other lines, or even extending the LRT all the way up to Sandalwood so that it can fulfill the full network relief potential it has.
A shopping mall, similar to Downton Brampton, has very little heritage significance, and I agree with you in that heritage should have very little to do with network decision-making on the scale of the Hurontario LRT, which is why it is a shame that heritage has been used as a lever to push the line, and 1.7 billion dollars - underground.