Ontario line was Downtown Relief Line only because it was planned to "relieve" Yonge Line. If Yonge Line had much higher capacity, we would not have been talking about a relief line. So, there you go.
Yonge subway is the busiest, highest capacity subway line in North America. For Yonge line to have higher capacity, it would need four tracks rather than two.
It is very obvious that Yonge line didn't need to be relieved all of last 50 years. Even now, after new signaling is done, Yonge line will still have some spare room left, howsoever small it may be. Relief line is mostly supposed to manage future crowd levels.
Things can't operate at 100% capacity for sustained periods. You need unused capacity -- clear space on trains -- in order for people to embark and disembark. You will never hit sustained 100% capacity utilization. Some spare room is the same as effectively full.
Yonge line was at capacity in the early 1990s (30+ years ago), at which point Premier Harris cut provincial transit funding and fares rose as quickly as ridership dropped. Transit ridership recovered in the 2000s, partly thanks to the ridership growth strategy under Mayor Miller. Since then we've increased capacity somewhat by adding a seventh car to the trains and improving the signals on the line.