Starting at age 50-60, one needs to review the RRSP spend down plan for the next 30-40 years.
Whenever the income spikes, well above what you will draw in retirement, is a good year for RRSP contributions.*
If they have rental property, it is prudent to keep a large RRSP room for balancing capital gains income over many years.
For example, if they sell a property that has $600k in growth (net of Actual cost basis) , taxed at 50% cap. gains rate = $300k.
If their business does not have another way (incorporated, trust structure, offsetting expenses) to reduce this, they need to declare $300k in income.
This would be an ideal year to "hide" the $300k income in the RRSP and defer taxes until retirement... Then they would spread out the taxes on it over many years, with a lower marginal rate.
But if the calculation about RRSP contributions is on income before age 70 under $90k/year today, versus a retirement income of $50k later, ** you are correct, that the advantages to putting $$s into the RRSP start to diminish... especially if they intend to leave an estate legacy after they die and hope to reduce taxes at that time. These are generally the years to max out your TFSA first, then decide how much to put into the RRSP versus non-reg. based on your RRSP size :income ratio.
*NOTE: Couples split RRIF income in retirement, so if they are drawing $50k / yr for two persons, that is actually only $25k/yr, per person, which will always be a lower average tax rate than paying income taxes on $90k/yr as employment income.