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.