I think the real win in the OL plan is showing Toronto that extensive elevated and automated (as well as more modest sized) metros are really actually quite a good option. If you look at the systems Spain built they generally followed this model and have been building stuff at a crazy pace.
If you look at how much we are spending on Eglinton and Finch, both projects would probably not be that much more to convert the at grade parts to an elevated OL style Metro (probably with shorter 4 car trains), however the quality of service would be miles ahead.
Obviously we need a Relief Line and have for some time, if we keep doing every major line as a full subway its going to take us a lot longer to build the big grid everyone wants. Plus, I think theres a strong case that having 2 smaller (but still fully grade separated) lines is better than having 1, you can get more coverage, more well distributed traffic, and maybe most importantly more overall redundancy to protect from shutdowns.