Please read more carefully.
I wrote that building subways and hoping for density to improve (which is a rationale cited for building the SSE) as a result has proven to be a terrible planning strategy. This is why, historically, subways have been built where there's density available to support them.
Warden was completed in the 60s. Kennedy and Warden in the early 80s. Despite this, density in the surrounding areas have remained low, and development relatively stagnant (Kipling is finally seeing legitimate highrise construction decades after they were built). That's why the RT is still generally quite suitable for the area.
As far as feeder stations go, the SSE doesn't compare to any that currently exist either (something pointed out numerous times in this thread). It's a 6km extension that will see only about 64,000 riders a day. Here are the ridership totals from some 6km stretches of subway across the city:
In addition, the Kennedy and Kipling extensions didn't cost $5 billion dollars. They didn't cost $4 billion either. For that matter, they didn't cost $3 billion each, $2 billion each or $1 billion each.
The total cost for both was $110 million (I believe that's 1970 dollars). The cost for Kipling was only $38.6 million. In 2017 dollars, that's approximately $681 million for both and $223 million for the Kipling portion. They weren't 6km from the nearest station either. From financial and geographic perspectives, you could find at least some sort of justification for them, even if they weren't being built in high density areas.
No such justification exists for the SSE.