They'll sell all 2M launch units, plus another 2-3M units at the holiday period. If they dropped 5M units into Holiday 2017 against aggressive PS4, XB1, and Scorpio sales and marketing they could be seriously challenged to sell out, so this is the best time to maximize 2017 units sold.
They can use the early adopters to effectively soft launch their online service with new features rolling out with MK8D and Splatoon 2, each progressively ramping up the scope of the service.
This also allows them to monetize 1-2-Switch on early adopters while being far enough out form the Holiday window where making it a pack-in to push sales wouldn't merit a loyalty program thing like with the 3DS and it's massive price difference, so they have a built in "price drop" without actually dropping the price this way.
Lastly it gives them an open field to market they system for several months leading up to the much more expensive and much more congested holiday season. I think you can make an argument for the Switch's portability and Nintendo's key demographic (the under 15's) as a prime audience to hit in the spring prior to summer vacations and family trips where good word of mouth can be spread to family and friends as well. The lack of launch software won't seem like a big deal as it'll have Zelda to headline and enough respectable alternatives to give everyone 2-3 games worth owning.
The real problem I see the Switch having is justifying the overall package cost. $300 for a system with no game, $60 per game, $70 for a good second controller, $80 for additional joycons, $30 just to have a charging grip for the included joycons. Sure, a good sized MicroSD isn't that expensive but with only 32 GB included it's yet another thing you need to buy for the device. It feels like nickel and diming the customer but instead of asking for nickels and dimes they're asking for $10's and $20's more than they and the industry at large has traditionally priced at. That doesn't get better by putting off launch a few months. In fact, the sooner they start moving systems and restocking the pipeline the sooner they can hit an economy of scale related price drop on all of this new tech of dubious worth that is, presumably, driving these inflated prices.
There are a lot of things about the Switch I disagree with but the timing of launch isn't one of them. Besides, if they'd waited until the holidays what would they have done with Breath of the Wild? Release on Wii U six months earlier and weaken a major selling point for the Switch? Cancel the Wii U version and undoubtedly get hit with a class action law suit they would surely loose? Or push the Wii U version back even further, requiring continued acknowledgement of that failure on their record? I mean, clearly the last is the best option but only because it is a collection of minor negatives without a single positive while the other choices carry at least one major negative.
