" for the simple reason that it costs much more to build a camera with a flipping mirror than without." Do you have a source for that ? - even for a per-unit marginal cost I doubt that's true. Sure you loose the mirror and the prism but you add an EVF which costs money, consumes power and generates heat that needs to be dissipated that a mirror doesn't. Mirrors and prisms are mechanical things that have been around for over two centuries - OLED EVFs are a decade-old or so tech. In the entry-level cameras that use a pentamirror rather than a pentaprism the cost of the viewing systems is even lower. Then there is the phase-detection AF which if goes on the sensor might as well add more to the sensor unit-price then having it as a separate unit, or it might not.

The reason Nikon and Canon have waited as long as it did wasn't because they didn't realize mirrorless is the way forward but rather because it involved big upfront R&D and manufacturing investments in new camera system and new lines of lenses which would take a while to pay off and in the meanwhile will cannibalize into the revenue stream of existing product lines whose major R&D costs were sunk and repaid long time ago, so it's no surprise that they held off as long as they did and it probably meant seeing some hard evidence about losing market sony/fuji/m43rds for one of them to cave first with the other being obligated to follow. The other thing is that dSLRs have hit an innovation and value ceiling about two generations ago, a dSLR bought five years ago in a given price point is about just as good as the current model - with a mirrorless at least the marketing departments will get a chance of trying to convince people that there is something essentially new here. The real sea change will be when we start seeing entry and even midlevel dSLR product lines discontinued in favor of mirrorless.