15k ppdph is achievable on the surface portion if you can get a train with 500 rider through an intersection on every 2 minute traffic cycle. 45 seconds should be enough time to get a 100m train from rest to move across the intersection. As long as trains can arrive before the intersection on regular 2 min intervals, it can be done. Ppdph has nothing to do with average speed, so rider experience on waiting at every red light will definitely won't be great. Especially if it's going to be slow.
Considering King can move 8k ppdph with the current configuration and Flexity cars. A streetcar through an intersection every minute with 135 riders gives 8100 ppdph. The only reason why capacity is no where close to that today is they don't have enough streetcars to run at 60-90 second headways. Transit Toronto mentions the Bloor Streetcar moved 9k ppdph in the final years. Of course a heavier longer train with a longer suburb traffic cycle though a wider intersection would reduce capacity. Hence a train every minute on Eglinton is not achievable.
This really shouldn't be an issue in the foreseeable future, Line 2 would be converted to ATO (hopefully) by 2040 along with a completed SSE. So Scarborough riders would most likely continuing their way through Kennedy while a 5min bus from Eglinton/Warden on the 68/968 to Line 2 will still be a decent option. I don't see why Line 5 will generate huge demands on the eastern section anything soon. Line 4 gain zero riders after 20 years of condo boom.