The reason Apple can do this is that they make their own hardware, so there's nobody else who needs to get paid and no reason to try and "differentiate". The downside of the Apple strategy is that hardware businesses are extremely difficult and expensive.
I'd argue that some manufacturers would be happy to make such devices if Google spent many millions in promoting them.
Skins - probably agree
lag-free - certainly
mandatory upgrades - agreed
"crapware"... one man's crap, etc... but in some ways I wouldn't necessarily mind on these if they were just separate bundled apps that didn't interfere with vanilla operation. I've only ever owned a vanilla nexus, but have heard nothing but horror stories about pre-installed apps that could never be removed - eating up space and taking pride-of-place on the home screens.
I'm not against bundled apps, but but bundled apps that can't be deleted and which interfere with basic operation out of the box, yeah, I'm definitely against those.
I kinda think everyone is reading way too much into this "X" branding. It's a codename. I'd bet some spare change in my pocket that it will be a Nexus-branded device.
I wouldn't buy the cheap one for myself, but I could recommend them to people who just want a cheap smartphone. I'd much rather recommend them a clean Nexus device, than a skinned custom-software one, with an unpredictable user experience, that also doesn't get upgrades.
All of these Nexus devices also need to have all of their upgrades handled by Google alone - just like the Chromebooks. In fact, this Nexus program should be handled almost like the Chromebook program. Nobody gets to touch the Chrome OS but Google.
I think these devices would become increasingly more popular thanks to the word of mouth of the people who actually know what they are and recommending them to all their friends. This would clean up the Android ecosystem a bit by having exactly one user experience across Android devices, and having a major portion of the Android ecosystem be upgraded on time and for a longer period.
As for this X-devices strategy from Motorola, they could use it as "iPhone-killers" - as hero devices of their ecosystem, like Nexus has been until now. But they need to be devices of highest quality, and they shouldn't try to also compromise on price, like they did with Nexus 4. I'm not saying that Nexus 4 suffered from it, but to be honest that strategy started to worry me a little because Google might want to get to lower and lower price points in the future, and forget about making the very best devices in the market. And I think that would be a very bad idea. Google needs to excite "influencers" in the tech world, and they can't do that with sub-par cheaper devices.
But with the X strategy, they can deliver these hero "best in the market" devices, while the Nexus program would allow for a sub-group of the Android market, to have devices from $100-$600, from different manufacturers, but under very strict guidelines, and with Google handling all the upgrades. Basically the Nexus program should become what Android should've been if Google could restart Android today.
tl;dr - Nexus = Windows/ChromeOS strategy, with one UX and all upgrades from Google for at least 2 years. X-phone - iPhone strategy; Google's hero device for the year, the one that's made to excite the tech-insiders, and push the ecosystem further, and the one that always comes with the dot-oh versions, not dot-one (e.g. 5.0, not 5.1).