The SSE as conceptualized is flawed in virtually every way. The Sheppard Stubway is flawed too.
The DRL as conceptualized is a critical piece of infrastructure that will have excellent ridership from Day 1.
Your biases are clearly showing. You can't just say that one project is flawless and another is completely flawed. I can think of a bunch of flaws with every plan, including DRL Short)(ie, lack of surface connections, the loss of the 501 and 504, station depth, density along the corridor, actual travel time, commuters' willingness to transfer, the fact that bloor yonge will become an issue again in the future, how many stations are actually present, diverts the problem farther south (people may choose to transfer a bunch from Queen to the Yonge line and take that to Union), where the train yard will be etc)
Likewise, I can see value in the projects you dismissed:
Sheppard (stub) - Shuttles people from don mills, prevents a dozen bus routes running along Sheppard for that 5 km stretch, decreases travel time (when compared to a bus), stimulates growth in north york, away from downtown, allows for transit-oriented development, allows for extensions, capacity available without worry, etc
Sheppard (full) - Connections to Wilson yard - allowing for a more efficient sending out of trains, reducing deadheading, freeing up space at Davisville. Connections to the Spadina Subway from the Yonge Subway - New, more efficient service patterns, redundancy in case of a closure at Eglinton, ability to access the west side of north Toronto from the east (York University). Completion of a line that has space for the future, is fast, stimulates the economy, etc.
SSE - Replaces an aging line, is faster than the aging line, reduces dwell time, removes a transfer, frees up railway corridor for regional express rail, stimulates growth at the STC, decreases trip time for the majority of Line 3 users (most Scarborough residents bus to the STC, and that's why that station alone sees a good 70% of the line's ridership), removes the need for a new or more complicated yard for LRVs/ICTS, will force Bloor Danforth line improvements, serve a mobility hub.
Do I believe the benefits of any of the above projects outweigh those of DRL short, long, and west? Absolutely not, but you cannot say one plan is completely flawed or isn't. Overconfidence in a solution is what got us into this transit mess and if we build an inadequate DRL, then you can say this city will be truthfully f***ed for years to come. If you're going to invest so much money in 1 project, do it right.