I think some of the technology issues involving size, etc. will eventually get fixed. The price may actually end up being less if the value proposition includes using modules in multiple devices, desktops, etc.
The software issue is two-pronged -- one, hardware advances so rapidly right now that I mostly give OEMs a break for bad support of older devices (maybe more nods to Apple, less to Android).
The biggest obstacle for both software and an Ara-like system with modularity is the economics of monolithic device release cycle. OEMs know they can count on a huge amount of sales as entire devices get bought every year.
It's hard to see Apple or Samsung giving up those economics for the economics of incrementalism or even bothering to support incremental component upgrades.