To be fair this isn't the end of the world. The transfer at Spadina is also 200m, and there are many systems in the world that frequently have transfers at that distance, London and Moscow come to mind. Since the line is elevated, there shouldn't be too many issues extending the REM B west if at all, and if Robert-Bourassa is connected to the Reso, the walk from Robert Bourassa to Gare Centrale would be quite similar to Bonaventure to Gare Centrale.
Also, Scar downtown? You can make elevated guideways downtown without scarring it, no problem. Reece Martin has an entire thread showcasing elevated rail in all sorts of different environments that don't "scar" the areas they're in:
Finally price, yes, one third of the price of the pink line is a big deal. The difference between an expensive and "better" line and a cheaper but "worse" line is that the latter can actually get built. You can have the greatest alignment for a transit line that you could ever imagine, but if after 10 years you can't find funding for it, your proposal is literally no different than doing nothing at all. Sure the pink line is arguably better than REM B, but REM B has people with money willing to pay for it, the Pink Line does not even after multiple provincial and federal governments. A worse line that gets built is better than a better line that won't.