Yes, it is possible for employers as well as for individuals, through their RRSPs, to substitute. If they're already at the point where they believe that their level of savings is adequate, they can decide to contribute less, for instance, to their RRSP or, through the employer, to the pension plan, which is also a key reason that the additional CPP contributions for the employees are made deductible.
On September 19th, 2016. See this statement in context.