There's really 2 types of people -- the responsible savers, and the ones that aren't.

The ones that aren't saving now won't be affected by a lower 401(k) contribution limit because they're ALREADY not using it.

And the ones that are responsible savers will surely remain responsible savers.  They just won't get to defer taxation on nearly as much of it.  But there are still tons of other regular (non tax-deferred) savings/investment vehicles for them to use (that they're probably already using in addition to 401(k)/IRAs).  

It would have modest impact on those people in between, that save more now because they get to defer taxes. But at the same time, they won't need to save quite as much because the contributions will have already been taxed.