Eglinton doesn't have the highest ridership east of Don Mills. The TTC is handling higher ridership volumes than are found on Eglinton with buses. Once all those condos on the Golden Mile are occupied, the Ontario Line will be running. I don't see the case for the Eglinton to be at capacity.
Let's hope so. However, the ridership level of a bus route is not always predictive for the future rail transit demand once. The latter can be boosted a lot if many riders switch from other routes.
Finch West LRT is easier to predict, since there aren't many riders who can switch from other routes.
Eglinton is a wildcard, a central and very long line that potentially serves a lot of origin / destination pairs, including those where neither the origin nor the destination is near Eglinton. But we can't know in advance how many people will actually change their routine.