Indeed, the demand for Eglinton West originating from Mississauga is a big wild card.
The scale of that demand is hard to predict, because the pattern is virtually nonexistent today. There is no usable transit connection between Mississauga and Toronto Midtown. Only people in really desperate circumstances would consider using the Eg West or Lawrence West buses for that purpose. Everyone else either drives or chooses their residence or employment to avoid such a commute altogether.
Once EWLRT connects to Mississauga Transitway at Renforth, a somewhat decent transit option for such trips will be created, and surely some people will use it. But how many, remains to be seen. This is still not a perfect option, with a transfer at Renforth and potentially two more transfers to/from local routes in the beginning and at the end of the trip.
With that in mind, I can think of Jane LRT being a branch of Eglinton temporarily, before the Eglinton West demand reaches its full level. And later, Jane LRT gets de-branched from Eglinton and extended to Bloor instead.
If Kipling-Islington is anything to go by, 40K PPD? Not terrible when you think about it. MiWay and Brampton Transit are getting better as time goes on, fare integration is inevitable, and development along the corridor is accelerating...a lot.
Honestly, when you actually think about how fast this trip may be, consider a trip from Square one to Dundas:
As is:
A. Via Eglinton
MiWay Express Bus to Renforth: 15 min
Line 5 to Eglinton-Yonge: 30 minutes (16 stations, similar to line 2, even though I think dwell times will play a role here)
Line 1 to Dundas: 10 mins
Altogether a 55 minute journey, round up to 1 hr for transfers.
B. Via GO:
You're SOL...to the point in which you're better off taking a GO Bus (which has huge risks due to the Gardiner). For the purposes of having a look though:
MiWay to Cooksville or Erindale: 10-15 min
Milton Line 35 minutes
Line 1: 5 minutes
Again, 55 minutes, round up to 1 hr.
So you're not actually saving any time using GO. Worse yet, you're transferring the same number of times, and adding risk to your trip since you're using regular city streets, and less frequent services.
As a mental exercise, Throw in 10K for the airport connection, and 60K as originally proposed, and conservatively, you're adding 100K people to the western segment of the crosstown (subtracted 10K from the estimate to account for the Eglinton West Bus connection at Mt Dennis). Considering the western segment of Line 2 sees about 250K users per day, and assuming the western segment of the Crosstown will see 100K passengers per day, you're looking close to line 2 capacities on the Crosstown west of the Allen, on trains that can carry 1/3 the number of people.