It's a bit ambiguous about if switch replaces 3ds or not, wherefore Nintendo deems it as home gaming system first and foremost! It says that it won't affect the sales of 3ds or buisness decisions regarding it!
Advantages of being a 3ds successor too : Game quantity and quality, since the home console first party games meet the handheld first party games and the somewhat good third party support of nintendo handheld console. Developing teams focusing into one device!
Drawbacks: One version of an ip, that maybe could not sell as much as the two versions ( one for the portable and the other for the stationary console ) of same ip would sell, attach rate on one product could not be enough to match the sales of a game in two systems.
Price, Handheld for obvious reasons, have to be relatively cheap, we all see what the launch price of 3ds ( 249$ ) did, albeit the dock doesn't have console hardware, it's a tv out and a charging cradle, hence switch could be surprisingly cheap ( keep in mind that there is no optical disk drive ).
Portabillity, switch handheld device is elongated so less portable than a new 3ds xl, while it's having a bigger screen just by 1inch! Apparently this is because of the detachable controllers and the tablet nature of the device , so it's not convenient for holding it with one hand and using a stylus for instance...
Drawbacks of a separate 3ds successor: A 3ds successor is redudant ( especially if that device would have the same cartridges with the same game ) when the handheld device of switch can obviously substitute its use, a 3ds successor could possibly cannibalize the sales that would come for the portability mode of switch!
That's quite slim and has great portability!
And could have an xl version like 3ds does!
