Wireless backhaul is the primary market for mesh systems being sold direct to consumers for self-installation and management. MoCA is still largely a service provider technology, few homes have network wiring, and powerline has turned out to be slower and more twitchy than hoped. Wireless backhaul is all that is left for 95% of the market.
However, if Comcast wants to get into the business of selling whole-house WiFi as a solution, they will be looking for something with different characteristics. They are going to want a multi-AP solution that uses the existing coax cable infrastructure whenever possible, that leverages existing skill sets when new infrastructure has to be installed (their installers know coax), that performs consistently in homes with widely different RF characteristics (no time or money for site surveys or trial and error placement of APs), and that doesn't generate a lot of service calls. A cloud-connected, high-density, MoCA enabled, wireless capable, access point sounds pretty attractive to me.
Plume is already part of the way there so I'd guess that the reason they made an investment was more to influence product direction than as an investment. I'm not even sure that Comcast would care about making it a Comcast-only solution as they have never seen much interest in competing through unique hardware. The money for them is in recurring service revenue and the more services you buy from them, the harder it is to switch.
Are there any other high density mesh solutions being sold into the residential consumer market today? I'd actually like to find one that does wireless, wired, and MoCA backhaul with 3x3 client connections and more proactive roaming. 2x2 is fine for my devices today but I can see it becoming a limiting factor in a few years and I hate being stuck on a good connection when a better one is available.