Re: Sprint is desperate, the other carriers are not.
Mechanically, the Project Ara was always going to result in a sub-optimal handset, in terms of size and ergonomics (you'd be using physical connectors many times bigger than the actual components that they connect), and not to mention dust ingress and the like. There were also issues such as matching a camera module with a DSP on the SoC to handle its output. An enticing concept, but just that that.
However, lots of people would like to slap on a bigger battery, a specialist camera, a good microphone, a keyboard or a gamepad to their phones - and they already do, via microUSB or Bluetooth - but again, these approaches can be suboptimal (eg placement of USB port). These addons are all supported in Android by the GreyBus standard, but currently the only sane physical connector is sadly proprietary to Motorola.
By contrast, the Essential phone just uses two power pins, and uses a wireless link to connect modules, limiting what the modules can do and presumably using more battery power than a wired link. Motorola's phones have been reasonably well received (even ignoring their Moto Mod connector), and the Moto Mod connector has been supported over a couple of generations of handset (unlike LG's modules) and has even sparked a crowd-funded project to make a Psion-style snap-on keyboard. If Moto can't make their modular system work in the market, what chance does Essential have?
The most popular modular phone was the perhaps the Nokia 6210 - its bottom-mounted docking plates continued into its battery bay, so new features such as Bluetooth could be added by buying a new battery with a Bluetooth chip in it.