That's a pretty massive IF. I don't really understand why you would take on that kind of risk at this time. Kjerstad will receive plenty of playing time this year and Santander doesn't have to be moved for it to happen. Mayo is 22 years old. If there's a lengthy injury to Hays, Mountcastle or O'Hearn, or if Cowser/Kjerstad have a brutal stretch in the majors, it would sure be nice to have an established, consistent big leaguer on the team to help weather the storm. It's not 2019 anymore and Elias, Sig etc. don't operate the way some people on this board do. Just because you have good prospects behind them doesn't mean you get rid of the established big leaguers. Keeping Hays, Mullins and Santander after 2021 is one of the smartest things Elias has done since taking over.
Or, given the volatility of reliver performance, the 1 WAR pitcher you think you are receiving regresses or gets hurt and you are clearly the loser of the deal. At this stage, I just don't see Elias trading an every day player of Santander's caliber for a reliever. It's bad business. Elias has shown that he is able to build a good bullpen and doesn't need to give up a top 50 hitter in baseball to do it. There are multiple solid pitching prospects in the upper minors in this system and plenty of guys that will be on waivers that are options in the bullpen. Let Santander play out the year, throw him a QO and take the pick. That is the smartest play which is why Elias will handle it that way.
I think there's a relatively solid chance it's a pretty big hole left in the lineup. Definitely a higher chance of happening than what you laid out in your post imo.