What happened to the Area-51m R Laptop?

It likely did not sell well enough to merit a new generation.  Only time will tell.

Apple isn't the competition -- the market for truly upgradeable desktop-CPU based notebooks  is a small one, largely served by Clevo -- which sells through boutique brands such as OriginPC, Falcon Northwest, Eurocom, AVA, Sager and many others.  The current model is the X170KM-G if you're looking for this type of system.  It does have a few major advantages over Dell's Area 51m systems -- including truly upgradeable, standard GPU cards.

If you're looking for a large footprint desktop, upgradeable replacement, Clevo is the last man standing.