Bethesda used to make great games, but the last great game they made was Skyrim in 2011, they unionized in 2022, and just recently they had some big layoffs.
CD Projekt made the highly acclaimed Witcher 3 in 2015, Cyberpunk disappointed on it's release in 2020, although it's reviews are mixed. CD Projekt unionized in 2023.
Blizzard have been in decline for a long time, they mostly focus on maintaining the cash cows games WoW and hearthstone. Although Diablo has is mostly well regarded(ignoring immortal). They haven't made any new ambitious game for a while. Their last attempt at making something ambitious was Titan, a cancelled MMO that they worked on for 6 years and supposedly spent $80 million on, cancelled in 2013.
The decline seems to begin well before the unionization, so I wouldn't say it's causal, it's more of a symptom of the company no longer being a place that can foster high functioning teams, where people feel like they are valued and working on something important.