Where is people getting the idea that they will release another handheld after this? They'll support 3DS over 2018 because there are games coming for it now in development, probably both theirs and third party, and they can't say it's over for that system while they want to sell many this holiday season along with Pokemon or whatever.
But didn't they say they were shifting towards unifying both console and handheld development of games? If any, they'll release a portable only, cheaper, smaller Switch compatible with every game, maybe with less resolution and size and not detachable controllers (IF they don't have motion tracking). They will surely also release a home only more powerful version down the line, if Pro and Scorpio do well and seeing as they've already done it before with New and DSi.
If that's not the case, if they're going to have still console games and different portable games, their software output still divided, I'm completely out of buying Switch ever, or at least until it's dirty cheap. The primary appeal to me is just having to buy one device to enjoy every Nintendo game from now on. Market it like that and it'll sell itself ffs.