Few military assets are more expensive than fighter jets. Our CF-18s are verging on obsolete and the costs of just keeping them in the air is ballooning as every single part of them are going beyond their operational lives.

We basically won't have an air fleet in 2 decades if we don't buy the F-35s now, and trying to refurbish the CF-18s while we hold out for the next generation or something will cost us tens of billions in refurbishment and maintenance fees alone while running an air fleet that can only keep up with 3rd rate air forces. Even if we can somehow hold on until a newer and more cost effective jet comes to market, any discount we can get from that will be nothing compared to the extra cost of keeping the CF-18s running. Not to mention the pure reduction in capacity in the meantime. We still have to patrol the north, and anything we use for that can't be spending weeks under maintenance between sorties.