I have a couple of comments.
First, the basic structure of the Canadian income tax system goes back to the 1972 tax reform, which was in response to the Carter commission of the 1960s. Carter recommended family-based taxation, but in response to pressure from women's groups or representations from women's groups, the government adopted the individual as the basic unit of taxation. That was intended to encourage women to participate in the workforce, as I understand it.
In terms of the basic underpinnings of the Canadian tax system, the individual is the unit of taxation. So I'm not sure that it's correct to say that it's based on a one-earner model with a domestic worker in the home. I'm not sure that's entirely representative.
The bottom bracket—the figures you presented were from 1988. Currently, that rate is 15.5%, so it's come down since that time, and there has been quite significant tax relief directed at lower-income earners, both through increasing the basic personal exemption and through attention to tax rates.
With regard to pension income splitting, I would just say that it could be viewed as recognition of the way many seniors did choose to live their lives, and it provides benefits for what was quite a typical family pattern from that day.