They wouldn't be the best selling games on PS4/Xbone/Switch if porting up was really harmful to sales. And MK8 is a Wii U game, Wii U is significantly weaker than the Switch, it's a port up. If they made, say, a Resident Evil game for Switch, then ported it up to PS/Xbox/PC, that would absolutely sell as long as it was a quality game. Obviously which way you convert would vary on a game by game basis, but there's absolutely merit to making Switch the lead platform for certain third party games.
"They wouldn't be the best selling games on PS4/Xbone/Switch if porting up was really harmful to sales."
This is likely a saying a poorly reviewed game (Fifa 21) sold extremely well, therefore critical reception doesn't have any impact on game sales. We know that it does for many games and we also know mega franchises also can get away with things which the average 3rd party title cannot. The question is not whether said up port can be successful, but would it be as successful as a game built natively for the respective platforms power profile. I mean are we suggesting that Monster Hunter Rise hasn't benefitted from being built around the Switch's hardware and thus looking amazing on platform? I thought we were all praising that a few days ago. If it benefits from that, why would Xbox/Playstation games not?
"And MK8 is a Wii U game, Wii U is significantly weaker than the Switch, it's a port up."
Compared to the gulf between a typical console generation or Switch vs PS4/X1, not really. Moreover the point is not about raw specs but about presentation & expectation. Mario Kart 8 looks on par with most of Nintendo's game built from the ground up for the Switch. Semantics aside, it clearly not in threat of looking out of place or dated amongst flagship Switch releases.
"If they made, say, a Resident Evil game for Switch, then ported it up to PS/Xbox/PC, that would absolutely sell as long as it was a quality game."
Sure, no one said it wouldn't. The question is which route of conversion would boast higher sales potential. To throw up my comment again "I guess it depends where the sales potential is higher. Porting up means said game is typically less fit for the Playstation and Xbox, meaning less sales on those platforms. Porting down tends to mean the opposite." I'm sure if Witcher 4 was built for the ground up for Switch it'd still sell a ton everywhere. But by doing that would they not also jeoprodise some of their playstation/xbox/PC audience, where it would no longer be the visual standout, where maybe even the ambition/scope of the world is reduced? Same can even go to Monster Hunter World, would it have been as successful if it looked like Rise but with minor improvements?
Its a tug of war between what audience you want to prioritise. The 250m of Playstation/Xbox/PC or the Switch? Referencing Mario Kart 8, artstyle and genre also comes into play. A stylised game like Minecraft dungeons does not need horsepower to bring its world to life & it lends itself well to Switch audience and play style. It can be scaled up without any issue. Also plenty of PC games look like crap but looks are not the focus (Valheim). The Resident Evil franchise typically leans towards the direction of realism and has a heavy focus on story telling, so if people feel this new RE entry looks worse than the remakes they were just playing, might that put them off?
"Obviously which way you convert would vary on a game by game basis, but there's absolutely merit to making Switch the lead platform for certain third party games."
We are more or less in agreement here but as mentioned before; this dilema is why I don't think it was an obvious thing for developers to just start making their AAA games around the Switch and why I'm not sure we'll really see this going forward outside of games Nintendo has paid exclusivity for
OP: I will go with Minecraft because it will not age and will have legs long after the Switch hardware has fallen of its cliff :p