Xbox 360 E was a way to further cut production costs, I assume. It was not an experiment or a test of some sort. Sure, Forza Horizon 2 launched in 2014, iirc, but what else? Rise of the Tomb Raider is a big one, but that's third party. Third parties can of course keep releasing games, FIFA 19 launched on the 360 I believe.
I hope the One will keep being relevant, I like that older consoles can still be supported even after new ones have launched. But given the limited success Microsoft's had with Xbox One, unlike the 360 which was a hit, I would be surprised.
I think that a console with no disc drive would primarily be a way to cut down production costs, not some grand experiment. People who would consider buying a console that late into the generation, especially a model that lacks the functionality of other also available models, represent a completely different audience from people who would be interested in Microsoft's next gen machine. It's not a good way to test the waters for anything, I don't know where that idea is coming from, because it doesn't sound reasonable to me - it's never going to sell huge numbers no matter what, because most people who want Xbox One are likely to already have one by that point. This is going to be a product for families and cost-conscious gamers, obviously designed to work in concert with Game Pass and perhaps game streaming.
Rise was technically published by Microsoft on Xbox, but even ignoring that, there were also Fable Anniversary, Screamride, and a bunch of XBLA games in addition to a separate version of FH2 and its spin-off Fast & Furious game (and let's not forget that Titanfall also happened and hit Xbox 360 just because of Microsoft's direct involvement). Quite comparable to Sony whose post-PS4 releases consisted of Gran Turismo 6, LittleBigPlanet 3, yearly MLB: The Show games, and also a bunch of PSN games. I don't think that Nintendo published a single Wii game after the launch of Wii U, and Breath of the Wild was the only Wii U game published after the launch of Switch.