"What am I, chopped liver?"  - PS4
PS4 actually did 20m this year.... but thinking about Switch maybe doing it next year, is what your wondering about? Also why is it more "insane" to do it in your 2nd year than in your 4th? imo doing so in your 4th year is more impressive than doing it in your 2nd year.

Look at the Wii, it sold really well.... but didnt last long, the same could happend with the switch.

Cause the PS4 had the slim couple of months before 2017 and the Pro even closer. It had so many bundles and deals like the gold edition. Also, a console is more likely to peak towards the 3rd/4th year with the price significantly lower. If the Switch get over 20 millions with a 300 price tag and no revision, it's showing that Nintendo can easily take it up YoY again with some marketing while the PS4 used almost every possible options in 2017. The only thing left is a bigger price cut which the 250$ with a game wasn't enough to beat the Switch on it's first year at 300$ with no games.

There is no way Sony will cut the price to 200$ like some are saying. If they do cut the price, it will probably be the same deals it was during the holidays which wasn't a huge success. 

Imo, I really don't think the Switch will follow a same trajectory as the Wii. The Wii was hated by gamers after like 2 years because of the lack of big games, it had huge droughts and was suffering from the non-HD argument while it was the big game changer in the world of television. The Switch will most likely follow the steps of a Handheld. I don't see Nintendo ditching the console after 5 years if it's their only console on the market, they won't have a backup so while the Switch is a success, they will make it last as long as possible. Also the concept of the Switch isn't a gimmick, you can't get tired of it like mostly everyone was with the Wii. We also see a more open consumer market on Switch, buying a lot of third party and indie games which will allow a continuous support while on Wii Indies were not really a thing and the only thing selling well was a box art with Mario on it or a family game.

It's too early to judge on the baseline of the Switch, I think we will be able to see it's trajectory a little more in the next fiscal year. But I really think it's heavily underestimated only because it's against consoles in their peak. If it was up against the first year of the PS4 and the xBone, we would have realise how well it is selling since right now, it's outselling both of them with strong competition.

The big argument is also the release of the next consoles affecting the Switch, but it will be the same as of right now, Switch will have an amazing catalog with a huge install base and a ridiculous price tag with a bunch of bundles and deals during holidays against new consoles with droughts and most likely around 400$ tag.

I don't think that Switch will have long legs. Just its hardware will limit its lifetime, because it's already struggling to run Nintendos own games.