The viability of purchasing RRSPs, obviously, and the economic and the tax advantage of investing in RRSPs would vary across the income distribution. I think it would be an overstatement to say the vast majority of them are concentrated at the very bottom of the income distribution. Women in the private sector, particularly those with higher levels of education, do achieve reasonable earning.
I think your point here is to express concern that within the private sector a significant proportion of women do not have RPP coverage and are likely to have earnings at which RRSP contributions are not feasible or do not make tax sense. And I would say that there is a significant proportion of women for whom that is the case.
I would add that there would be women farther up the distribution who are making RRSP contributions as well. There's a range.