The Cardinals biggest need is pitching, not another bat
Just look at the Cardinals start of the 2023 season. Sure, they've had a few down games in their last few contests, but they are showing themselves to be one of the best lineups in all of baseball this season.
Their outfield has five great options in Lars Nootbaar, Jordan Walker, Tyler O'Neill, Alec Burleson, and Dylan Carlson. Nolan Gorman has cemented himself as a great DH option. Brendan Donovan continues to take his offensive game to the next level. Juan Yepez and Masyn Winn are down in Triple-A and could impact the club at any point this season.
The Cardinals have superstar bats in Paul Goldschmidt and Nolan Arenado. They added a terrific third bat in Willson Contreras. These young bats are under team control for a long time and will lengthen the St. Louis lineup in such a way that there is no easy out in the lineup.
Of course, pairing the Cardinals' MVP candidates with Soto would have been insane to watch, but it probably would have left them with a great middle-of-the-order but weaker supporting cast. I'm not sure trading for Soto would have made their lineup that much better in the long term due to the assets they would have given up.