The only thing I can really add is that the decision to integrate was made by Parliament in 1966. They did what seemed best to them at the time.
It is certainly prudent to occasionally take a look at decisions we've made in the past and see whether they continue to be what we want. However, the pension plan is just one small part of the whole business of your retirement income. You have CPP, you have your work pension plan, and then you have your personal savings. It was never the intention that just the one work pension plan was going to pay for everything for you after retirement.
Some of the questions we get are related to that: my pension is not sufficient to pay for my needs. But that's not a fault of the pension plan, nor of its design. The decision was made that this was how much we were going to pay for.