When the fighter's primary job will be patrolling Canadian airspace, the F-35 is really useless. Even a Russian Su-35 aircraft is overkill.
Much air-space defense involves going somewhere and "showing the flag." This makes you an easy target. You don't want your super-secret ultra-expensive stealth aircraft to fly up to a Russian radar aircraft at close range - it would tell the Russian's exactly how to detect the aircraft. Instead, the best option is to send an inexpensive fighter to say "go away" and "show the flag". If the inexpensive jet gets shot down, then you send the expensive stealth aircraft with "shoot-on-sight" orders.
The first problem with the F-35 is that even if you purchase the F-35, and believe it is good, then you still need to purchase an inexpensive fighter jet to replace the CF-18.
The second problem with the F-35, is much of Canada's interdiction duties require slow moving planes. If something is happening in the arctic, or off the pacific coast, then it is necessary to send a fast aircraft out to find out what is going on, and then make them wait until backup arrives. The CF-18's have had problems in this role. If you fly to slowly, you can evade them. (The CF-18 has a minimum airspeed, and the F-35 has worse low-speed aerodynamics than the CF-18.)
Most importantly, the F-18's were picked because they have two engines. If one engine failed in the middle nowhere, and most of Canada is untamed wilderness, then the second engine can get you back home. The F-35 is a single-engine aircraft (like the F-16). A two engine plane is much more survivable in Canada than a single engine plane.
Bottom line: Even if you like the F-35, we still need a different fighter. The F-35 fighter is not the right aircraft for most day-to-day tasks. The US understands this. Both the US Navy and the US Airforce plan to keep the older F-18 and F-16 fighters around for non-stealth applications.