I agree with being open to taking Richardson, but I don't think the argument for doing so lies in the backup situation. Really I don't think that plays into the analysis at all. You can find capable backups, likely better than Sudfeld, all over the open market in free agency. Their mistake in that department was believing one of Boyle or Blough could be competent enough to not lose the job in the preseason, and they're lucky that Goff didn't get hurt, because the mistake probably would have burned them. If you're playing a backup in year two or three of a rebuild though, you're likely going to have a bad time anyway. It's not until you turn into the offensive machine that the Patriots of yesteryear were, or the 49ers of today are, that you can plug-and-play a semi-competent QB like Matt Cassel or Brock Purdy and just keep on rolling like nothing changed.
If they drafted Richardson, I wouldn't be all that surprised to actually see them still pickup a veteran backup QB on a one-year deal, like Teddy Bridgewater, Gardner Minshew, or Andy Dalton, and have Richardson start the year as QB3, not even dressing for a number of games. It's not worth stunting his development and forcing him to play before he's ready, especially when he's likely not going to perform better than any other backup QB anyway.