1) The issue should first be fairness, not whether or not it contributes a substantial amount in tax revenue. 
2) As a public service employee (retired), I have no problem with converting from a defined-benefit plan to a defined-contribution plan, with one caveat.  
If public service employees have to shoulder the risk, they also should be compensated for that risk.  Just as insurance companies levy higher rates for policies and policy-holders with greater risk, so should employees be able to levy a higher salary for moving from a no-risk plan to a significantly higher risk plan.
3) Self-employed people say they pay double on CPP.  That is false.  Both employer and employee pay into CPP.  Self-employers make a personal choice to be the employer and hire themselves as their employee.  Their choice makes them, by default, responsible for both components.  But it is their choice, not that of society - even when, as in many cases, they provide a welcome service.