More four and a half year, but the more important point: WiiU came out with a power consumption of 30-35 Watt, while the Xbox One X came out with one of 150-170 Watt. Power consumption is the main variable for portabilitiy, as it both decides about battery life and needed cooling constructions (and therefore form factor).
it's not even just wattage that's the difference here, look at the WiiU form factor, I mean you're talking about a very very small system which houses a DVD drive, if you were to take the drive out you wouldn't be too far off the size of the Switch, possibly double it at most, so in 4 years you are looking at making a portable machine around half the size of what you had... now picture that with an Xbox1... I don't even think the game gear would get a look in for size if you tried to sell consumers a handheld half the size of that system.
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