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All I ask is that the darn thing work, plugged in, with a dead battery. I often skip console generations. A user replaceable battery would be better, but I don't see it happening.
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... If they made the battery easy and quick to replace it may be a little better, but they would have to sell replacement batteries so you could have a spare on hand to switch out and I doubt that will happen. They'll probably just have an integrated battery like most other wireless things these days.
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I think there is a fair argument for both integrated batteries and removable. Integrated is cheaper overall (part of why it's popular at the design level), but with all the bad press the Galaxy Note 7 has been getting, making the batteries easier to replace if they start acting up also makes sense. The Note 7 is the most recent and most visible case, but lithium-ion batteries have been blowing up or catching fire occasionally for years.
At this point, it's probably too late for Nintendo to pivot on the 1.0 hardware design; their stated release is only about 5 months away, which means units/parts are already being manufactured. If I were a betting man, I'd put money on integrated with some extra voltage regulation to keep from turning the Switch into the Mario Galaxy Note 7.
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Whether they gain and keep momentum will be entirely on the hardware; battery life on typical usage, battery on heavy usage, downtime for charging / can you use it while charging, wireless latency between all the bits... let's hope they were paying attention to the performance sins Ouya committed with their wireless controllers. They'll sell a honest pile of units no matter what because of the new Zelda title, if nothing else. (I'll even admit that it looks excellent, and I'm a pinched tempted... and I haven't owned a console in ~20 years)
I do wonder about the software architecture as it relates to the multiplayer-while-portable parts of their little sizzle reel. If there is a typical host/client arrangement, someone is gonna get hosed on battery life; if the runtime load is distributed betwen all participants, what is that going to do to overall latency? Some of this will probably be case-by-case basis for each game, but foundation choices made by Nintendo will definitely influence, overall.