Overwatch development has basically stopped; everything is on hold waiting for Overwatch 2. Because they announced Overwatch 2 in 2019, with a strong implication it was already well along the track, and because they announced they were stopping all new content for Overwatch in 2020, a lot of people expected Overwatch 2 to drop sometime in late 2020. Well, it didn't happen. And in fact, communication about Overwatch had dried up, and the lead designer (the much loved Jeff Kaplan) has left. At this point, people are guessing Overwatch 2 may be landing in 2022 - which means ~2 years of Overwatch being abandoned.
In context, there's no surprise Overwatch is haemorrhaging players. They've as much as told people "go away, Overwatch is dead, check back in 2022 for the sequel".
Meanwhile on the World of Warcraft side, the Warlords of Draenor expansion (released 2014) was badly bungled; many headline features were scrapped, and the pipeline of content that keeps subscriber numbers up between expansions wasn't there. They (very obviously) panicked, and threw EVERYTHING at the next expansion, Legion (released 2016). It was quite well received, and it was clear player numbers had recovered. The problem is that the following expansions (Battle for Azeroth, released 2018, and Shadowlands, released 2020) have been pretty underwhelming. But with a two year expansion release cycle, and two underwhelming expansions in a row, that lines up with a sharp drop in active players over the last three years.
Also don't forget the announcement of the Diablo mobile game; some PR missteps made the fans think it was a Diablo 4 announcement; when it turned out to be a mobile game it was, basically, worse than nothing.
There's also the misteps relating to WoW Classic. It had been a perennial fan request, but Blizzard had always refused, at one point infamously telling fans that "you think you want it, but you don't". Well, they finally released WoW Classic, and it turned out the fans really DID want it. Without that the collapse in WoW subscription numbers during BfA would have been much worse. But when you think about a team that struggles to make compelling new content, then argues with fans about whether there's any appetite to revisit old beloved content, then finally discovers they were wrong and there is...well. It raises some serious questions about whether the current team understands what the player base even wants. Because on some level, you can look at the last few years as Blizzard saying "you guys are wrong, you don't want Vanilla back, you guys want BfA!" But no, the player base wanted Vanilla, and did not want BfA.
Finally, there's been a long, long series of horrible esport misteps, from the re-written EULA that ensured the Warcraft 3 remaster couldn't possibly become an esports staple, to their successful efforts to ensure that if they couldn't run a SC2 esports scene nobody else could either, to some very puzzling choices around Overwatch team and tournament structures, to the killing of of HoTS, to their repeated awkward attempts at creating an esports scene in WoW, despite the game being unsuited to it and the player base having no interest. (Seriously, between Startcraft 1 and DoTa, Blizzard had the modern esports scene handed to them on a plate, and threw it away in an attempt to gain control.)
29% decline? You don't say...