The F-15EX is designed to be a stand-off missile truck that works in tandem with a network of forward-deployed stealth jets. This compensates for the relatively low ordnance load-out of stealth jets. A stealth jet can designate and cue a target from the front using an F-15EX as a missile launcher safely in the back since it is non-stealthy.

The F-15EX isn't a particularly survivable aircraft on its own. It is a plane you'd only use if you have stealth jets and you plan to do air combat at scale. It augments the capabilities of something like an F-35 but isn't something you'd add to your fleet unless you had a lot of stealth aircraft.

The F-15A/C is designed to act as a missile truck, but the F-15EX was specifically designed to be a multirole strike fighter. You can operate it without putting 20 AMRAAMs on the pylon, or without an F-35 datalinked to it, and many countries do. The flexibility and heavy ordinance are the selling point at home and abroad.

Canada could very well operate the F-15EX without a fleet of stealth fighters. The F-35 is not an interceptor platform in any sense of the word, in an "all out" scenario like you're suggesting it's doubtful they would be any better than the CF-18 they have today.