IMO they are only pretending to be serious. This thing will never be built (at least not within any of our lifetimes). Even the most pro-highway people could easily find many other projects that offer way better value for money (subsidize trucks on the 407 anyone?). As another poster wrote on the 401 tunnel thread, it's purely political theatre.
Trying to stay on topic, as an alternative to burning cash on a 401 tunnel, I'd rather see a Sheppard subway from Pearson to Malvern. More realistically, why is more attention not given to extending Sheppard west to Jane? I bet Oakdale Gold Club would love to sell off some land at the SW corner of Jane/Sheppard which could become super dense.