My view on a supplementary CPP is that it can be workable if it's a defined contribution arrangement and it's voluntary--or at least you can opt out if it's automatic enrolment.
There are different iterations of a supplementary Canada Pension Plan. One, some people talk about expanding the defined benefit guarantee under the current CPP, or increasing the YMPE, or increasing the benefit amount. The other is having a defined contribution arrangement.
If it's the first--in other words, if it's an expansion of the current CPP--you have to make it mandatory for everyone. If it's going to be defined contribution accounts--