First, why do you think that? What magical thing happens at Steeles?
Second, who is this "we"? Do you mean that York Region should be kicking in the municipal portion for the Sheppard or DRL lines?
EDIT: third, fear not -- as you'll see from earlier posts in this thread, the mega-bus terminal you call for is, indeed, on the drawing board for Steeles.
Sheppard and the DRL are needed. Sadly, it looks like Sheppard will never happen -- Giambrone the Subway-slayer is on the case. However, it does not make sense to play them off against other needed transit improvements.
I recently moved from Montreal and we did the same thing that your about to do. Look at the map:
Both Transit systems look similar.
1-On the east side of the orange line, the last station on the island was Henri-Bourassa. They added 3 Station in Laval (Cartier, Concorde and Montmorency)
Even with the blue line that connects both Orange lines, the trains were packed after the 3 stations in Laval. When the train arrived in Montreal, nobody could get in at all.
The green line used to be the busiest line but now it’s the orange line.
2-They saw it coming and they built a third platform at Henri-Bourassa where the train would leave from Montreal only and not from Laval.
This is what happened. Those who couldn't get into the trains coming from Laval went to this platform but they are sharing the tracks. Even if you had a train every 2-3 mins, it was still overcrowded.
3-Now they just realized that the only way to fix the problem is to either built LRTs or more subway line to relieve the pressure of the orange line. The best solution would be an express track like in New York but the Metro wasn't built for it and the cost would be like building a brand new line.
4-Here in Toronto, I heard the TTC saying by upgrading the signalization; the yonge and spadina line could run smoothly. I've seen it in Montreal and I predict that the same thing will happen.
We had the blue line and it did nothing. With an incomplete Sheppard line and no DRL built at the same time, the yonge-Spadina line will be a nightmare and I'm not even speaking of the subway platforms that are way to small.
5-I'm not saying they shouldn't do the extension but the transit system needs to be upgraded so that it has the ability to take some pressure of the Yonge line. The LRT could be a solution like the Jane and Don Mills line but as long as they connect together downtown and underground. Of course, I hope they go faster than 40 Km/h.