Undertale sold well enough to the point that it granted him financial independence and allowed him to work on Deltarune at his own pace.

Maybe even to the point where he got sidetracked too much, because Chapter 3 and 4 definitely took much longer than he expected. It was supposed to release with Chapter 5 after all.

I agree, except that Nintendo has somehow managed to actually be the exception to that. Tears of the Kingdoms mechanics (fuse, ascend and rewind) are the type of mechanics you would have seen some flash or indie game a decade ago and would have been wowed by, except seamlessly incorporated into a giant open world action adventure game. For all their shitty business practices and shovel ware Mario party games, they have somehow managed to leave some actual free design space for creatives to mess around in at the base of their biggest franchises and that's more than I can say for like all of the rest of the industry.