the OP is mostly right when looked at logically. Even if 3rd party devs WERE going to jump onto the Nintendo bandwagon again, they wouldn't do it immediately. They'd wait until the Switch had been out for a while and was proving that it was killing in sales.
also there sort of is a fallacy around here where people seem to believe that Nintendo DOESN'T have third parties at all- they certainly do when it has come to handhelds in pretty much every generation.
Particularly the big Japan developers will undoubtedly be pushing big software onto the Switch (Capcom, Square Enix, etc.). I think its fairly likely we see some other big stuff pop up too, I mean its a pretty positive sign that Bethesda is putting their FIRST ever title on a Nintendo console with SKyrim remastered in the Fall. and that EA is at least putting their newest Fifa game on the Switch.
Also even if there were some secret big third party announcements for the Switch from a few Western devs- we totally wouldn't hear about it until E3.
basically- I wouldn't honestly worry about 3rd party support (or the lackof) until the system has been out for a year. And at minimum until the E3 annoncement dust has settled.
also realism has to come into play. the Switch is partially a handheld console. Any game developed for the system or ported to the system needs to work equally as well (essentially) docked as it does in handheld mode. So some big Western games (like the new Red Dead and, say, GTA) may not realistically be releaseable on the Switch if they can't perform well in handheld mode (or cause the battery to drain in like 30 mins).
So I guess the point I'm making is A) its way too early to judge third party involvement OR for third parties to activiely decide to jump on board, B) Nintendo DOES certainly have 3rd party devs from the handheld angle (like Capcom with Monster Hunter), and C) its unrealistic to expect some portion of big Western open world games to release on the system if they're too draining for the system in handheld mode (which a big part might involve devs not wanting to put in massive work to optimize a port to the extent it would take to go from, say, Red Dead 2 playing on the PS4 to it playing on the haneheld mode of the Switch)