An alternative view, just to play devil's advocate.
Osuna's value as a reliever might be at his highest right now. Teams who buy elite relievers are generally those that believe they have a shot at making the playoffs. This doesn't get more obvious than at the trade deadline, with only 50-60 games more to play. The only relievers that I can remember getting traded for a haul in the offseason have been Kimbrel and Chapman. We just saw that Chapman just got twice that value, as has Miller and the past, and Miller a few hours ago. Hell, even Mark Lowe was worth two prospects, and he's a total shitbag now. Elite relievers are different, but I think teams would rather pay now while a BP arm is pitching at their best rather than risk it next season when they might completely gascan as many do.
Osuna is fantastic, but maybe a team trading for him in the offseason weighs more heavily the risk that he might get injured, the possibility that their team might not be fully in the playoff hunt come midseason, and the risk that Osuna's walk rate will skyrocket to 5/9.