There are other sections as well in NS, PEI, NL and QC.
The TCH isn't a 'Canadian Interstate'; it's just a designator on a bunch of provincial highways, with multiple routes in some provinces. A number of provinces obtained federal money to improve their sections of the TCH. In fairness, maybe Ontario does too.
No doubt twinning a route through northern Ontario would make for a safer, less stressful drive. It is arguable whether it would have a significant economic impact. There are a couple of sections where Hwy 11 and 17 are co-aligned (so no alternative route). A sample west of TBay shows a 2021 Average Annual Daily traffic count of 2000 vehicles (all types, both ways). A similar point at Nipigon shows 5600. So not huge volumes, let alone dividing that between 11 and 17.
Mush like Van Horne found with the CPR, I suspect building highways in northern Ontario is a lot more expensive than out in the prairies. Major highway expansions happen in 10 or 20 km sections every couple of years. Even if Ontario opened the taps tomorrow, nobody on here will be alive to see it happen end-to-end.