Did you watch him at the 2020 French Open, like the epic with Schwartzman? Thiem did not go into decline immediately after winning the 2020 US Open. His decline started when Dimitrov beat him in the fourth round at the 2021 Australian Open 6-4, 6-4, 6-0, when Thiem had been the clear favourite to win going into the match. Some people might even say going two sets down to Kyrgios in the previous match, before Thiem came back to win in 5 sets. Thiem's results then went into decline for the next few months (compared to 2017-2020) until the wrist injury in June 2021.
It wasn't a disaster at all.
Gaston played very well, and had beaten Wawrinka. Did you watch that Thiem vs. Gaston match?
Thiem lost to Schwartzman in an epic. Schwartzman's 7-6, 5-7, 6-7, 7-6, 6-2 win stopped the highly anticipated Nadal vs. Thiem semi final.