$5 million for a club like chelsea is not a lot. They are taking a flyer on the guy. They can make up that money with loan fees and a low transfer fee if he becomes a bust.
Sure it could. Miazga lead the team to a top 5 defense playing next to a guy so slow that he set the team's single season record for yellow cards entirely through professional fouls. Chelsea doesn't shell out $5M for a guy who isn't dominant in MLS. I wouldn't let a few bad months in France taint your memory of the guy, he was a pleasure to watch play here.
And when he left he only established himself as a starter for 1 season. And played only 34 games at the pro level. The kid was good but not an MLS best. It was obvious that he still needed more experience