Tage Thompson only broke out last season.

When you look at teams, they have competitive windows. Looking over a 5, 10, 15, or 20 year stretch, every stretch of 3-5 seasons varies in importance. Tage Thompson would have helped the Blues from 2022-2027+, but he would not have helped them from 2018-2022. That was when their window was open.

If they knew Thompson was going to be as good as he is now, they would have never traded him. They would have given up another 1st round pick, or a different prospect. Same deal with Josh Norris in the Karlsson trade (don't mistake that as Norris = Thompson). With that said, I'm sure everyone involved in the Blues org' and every fan still does this trade 10/10 times. Winning a Stanley Cup is nearly impossible. ROR was perhaps the most important piece of their run, and they don't win the cup without him.

You would have to have a crystal ball and know that they would somehow have peaked a rebuild or retool around Thompson that resulted in 1 or more Stanley Cups.

This isn't like the Karlsson trade where San Jose gave up Norris and the pick that became Stutzle, and only has a Conference Final appearance to show for it. Karlsson is amazing right now, but it's arguably a detriment to their team because it hurts their draft position in a rebuild. If you're St.Louis, it probably stings to see how good Tage Thompson is, but they have a Stanley Cup to help them get over that sting.