Pro: The Cardinals need more stability in their rotation, and three starting pitchers can bring that
Mozeliak's quote "We have six starting pitchers" this past offseason has been used against this club so many times since the words came out of his mouth. Sure, they had six options, but it became clear early on that they were not good enough to win games for this team.
The only defense I'll make for Mozeliak here is that, although many called out their lack of starting pitching before the season began, it's gone far worse than anyone could have imagined. Some thought Adam Wainwright would be bad this year, but no one expected him to have one of the worst seasons of all time. Jordan Montgomery was good in his final stretch of starts for St. Louis, but he started off a bit slow. Miles Mikolas did as well. Steven Matz had to be removed from the rotation and then got hurt when he figured things out, and Jack Flaherty was healthy but inconsistent.
The fact that all five of those things happened at once is a major reason why things fell apart. They probably could have weather one or two of those issues, but not all of them. So, I get why they would want to go into 2024 with five reliable options rather than a bunch of "what ifs" in the rotation.
Mikolas and Matz are very capable options for their rotation, but neither can be relied on to be "top-of-the-rotation". Dakota Hudson, Zack Thompson, and Matthew Liberatore are great depth options, but there is increased risk in slotting them into your rotation plans before injuries even take place.
Signing and trading for three proven MLB arms, ones that don't have the same question marks that guys like Flaherty and Wainwright had this year, put the club in a much better position to compete on a nightly basis in 2024.