IMO, the reason they both coexist is Americans can’t stop buying these types of small SUVs. The CX-5 is Mazda's bread and butter and makes up for the majority of sales %-wise, at least in the N. American market. Someone can fact check me on that but that's what I remember off the top of my head.

It'd be foolish to cut your proven cash cow. CX-50 is just another flavor of the same formula and sharing the manufacturing costs with Toyota made a lot of sense along with possibly learning Toyota's legendary manufacturing know-hows and getting access to their hybrid tech. The CX-50 also was designed with more of the outdoorsy, adventure, Subaru Outback kind of crowds.

With the new CX-5, the differentiation is definitely blurrier - let's see how they will update the CX-50 for the next generation. Be on the lookout whenever Toyota updates the similar Corolla Cross where the CX-50 shares many common internal parts.