Would it though? It would probably sell pretty well to people that hang out in gaming forums. But, does that average Switch owner really care about playing that game? How about the average Fallout fan... do they want to rebuy a game they finished years ago? Do they even own a Switch? Certainly some of them would buy it. But, how many, really?
Easily, ports are generally cheap, and Fallout is very popular IP, far more popular than Doom and Wolfenstein for instance, games that are actually ported to Switch. All that you could said for Skyrim also. Fallout 3 port would most likly had better sales on Switch than Doom and Wolfenstein 2 have.
Fallout 3 is generaly more popular game and had better sales than New Vegas. It doesnt need to be on Skyrim level, but if Bethesda is so happy with Doom and Wolfenstein 2 sales on Switch that they confimed that next Doom and Wolfenstein games are coming to Switch, they would be also happy with Fallout 3 sales on Switch (that would again most likly sell better than Doom/Wolfenstein2 on Switch).
Point is simple, if they cant port Fallout 4 on Switch in order to have accetible performanse and look, and they think that Fallout 76 is not best suit for Switch, the obvious choice would be to port Fallout 3 on Switch and offer main Fallout game in full handheld mode. In text they even said they having fan request for port Fallout 3 to Switch.