Ridership totals is also a misleading figure for rapid transit anyway, because the three lines mentioned would all have different commuter patterns. The Relief Line (when built to Sheppard) was projected to exceed both YUS and BD in ridership at morning and late afternoon peak hour. Meanwhile, both YUS and BD will have more mid-day, afternoon, and evening ridership during the non-peak periods, which when added together, increases their total ridership numbers to surpass that of the Relief Line.
For me, it's a moot conversation. The Relief Line/Ontario Line will serve its purpose by improving commutes for those who use it, and importantly, expands rapid transit coverage to larger parts of the city. The only real question is if the Ontario Line as proposed by Metrolinx will have the capacity to handle peak hour ridership on day 1 (and on day 10,001).