I think in 5 years, the SRT would be on its last legs. I wouldn't be surprised if they only keep the SRT open during weekdays from 6am-8pm. Honestly the 903 express bus takes about the same amount of time between STC to Kennedy than to have to wait and transfer to the SRT. At lighter times, shutting down the SRT and extending express buses to Kennedy isn't a bad idea. They won't need a new garage as long as they can keep the SRT running in rush hour.
As to why does it take 12-15 years to build a friggen subway, only Doug Ford can tell you. With only 3 stations, they shouldn't screw up so badly. If they can tunnel within 2 years (2021-2023) and give 6 more years to construct the stations, this thing should open in 2029. Meanwhile tunneling, they can relocate utilities. Hopefully ML has gain experience from the Crosstown that they need to start digging once the contract is awarded and not wait 3 years to relocate utilities and demolish stuff.
If the 2 contracts go out as planned, there is hope this will finish by 2030. Funding is usually the biggest hurdle that hopefully we shouldn't have to worry about.