Switch really doesn't need replacement until 2023 at the soonest. Killing the goose that lays the golden eggs just to play graphics catchup with companies they can't outspend in that department would be about the dumbest thing Nintendo could do.
People don't buy the Switch for graphics or new AAA multiplats, so long as it gets a steady supply of quality first party content, it will continue to sell.
I think a March launch for the next console from Nintendo is a smart move. Get the initial rush of sales at launch, have a decent sized game or two before the holidays, then have a heavy hitter for the holidays will help get the momentum going.