I honestly think that nintendo is on the right track of decision here. On the marketing point of view, the Luigi's Mansion remake would sell more Due to higher userbase on the 3ds compared to the switch. if anything else, users would completely buy the game on a currently existing console they have or buy a new 3ds to play this game, buy Luigi's Mansion 1 and 2, play it on the same console rather than buying a switch.
Also, i'm quite sure that most of the 3ds users dont mind the graphics. The port was clean and acceptable for the small handheld. As long as It is as good as luigis mansion dark moon and not as bad and heavily pixelated like the xenoblade chronicles, this game is perfect. Comparisons come to the small number of people because they have a console to compare it to. (Small number of Users who doesnt have a 3ds, or users that want it to the switch)
Honestly, i want it to the switch too. But considering the facts i mentioned above, i know that it is more practically correct decision to put on the 3ds. It will sell more to the 3ds more than it will on the switch.