Nintendo launching their newest hardware with a mainline Zelda, mainline Mario and a Splatoon game is of course the best thing ever for fanboy wet dreams but for everyone else it literally makes no sense to not space out the schedule a little. Nintendo won't care if you buy the Switch at launch or a few months later as long as you buy somewhere around launch until the end of the holidays. More importantly people don't buy your console because of the "amazing" games at launch they buy your console for a hopefully promising future release schedule after launch. Zelda releasing at launch or a few months later won't make much of a difference as fans will buy that and the Switch when it comes out and if it is really close to launch most of them will even buy the hardware at launch anyway.
With Mario, Splatoon and a hopefully healthy release schedule afterwards it will already be an excellent first-party launch line-up. That gives games Nintendo probably had a hard time even getting on their console in the first place still a good chance to succeed - be it Skyrim, Mario X Rabbids RPG or whatever else is coming out at launch from a third-party source. And launch title success is one of their best chances to convince third-party publishers for future continued support. Breath of the Wild releasing as well, though? No one will ever touch these other launch games if Nintendo still has a healthy release schedule ahead (which they should). Also, while not completely comparable to Nintendos situation here there is still a reason why most successful publishers don't release two major IPs at the same time. And when they did it it mostly backfired hard (looking at Titanfall and COD as a recent example).
With Mario, Splatoon and a hopefully healthy release schedule afterwards it will already be an excellent first-party launch line-up. That gives games Nintendo probably had a hard time even getting on their console in the first place still a good chance to succeed - be it Skyrim, Mario X Rabbids RPG or whatever else is coming out at launch from a third-party source. And launch title success is one of their best chances to convince third-party publishers for future continued support. Breath of the Wild releasing as well, though? No one will ever touch these other launch games if Nintendo still has a healthy release schedule ahead (which they should). Also, while not completely comparable to Nintendos situation here there is still a reason why most successful publishers don't release two major IPs at the same time. And when they did it it mostly backfired hard (looking at Titanfall and COD as a recent example).