I also work on a 27" iMac. The idea of giving it up for any reason would be unacceptable to me.

Although Apeture will go unsupported down the road, your current version, nor the next release aren't going to become totally obsolete and quit working anytime soon. When Apeture eventually get to the point that it begins to be a problem, you can switch over to an Apple version of Lightroom for much less than what it will take to change hardware.

Will the laptop be your primary editing station? Are you comfortable switching everything over to a new operating system? Will not having that great 27 inch screen available for editing be just a trivial concern?

If it were me, I'd stick with a MacBook, continue to use Apeture until doing so becomes an annoyance, and then make a software change. Of course my iMac and I have a deep, longtime relationship, so I may be biased!

There is one other use for the pop-up flash. It can be used to trigger an off-camera Speedlite, when that Speedlite acts as a slave. There is a setting on the camera that will handle this. When in this mode, the pop-up will not produce enough light to affect the shot, but it will trigger the slave. The one drawback to this approach is that, even though the slave may have HSS capability, the pop-up doesn't, so you won't be able to use a shutter speed faster than the shutter sync speed of your camera. If you aren't shooting faster than 1/200 or 1/250, this approach works fine.