https://ironforge.pro/population/classic/overall/
As you can see in the WCL Data(which is incomplete blabla) TBC brought Classic raid activity nearly back to peak 2020 pandemic levels. Blizzards subgraph has exactly one spike in 2021 and its right before TBC Classic launched. Also from the WCL Data you can see that the Classic player activity after it settled in has been extremely consistent so a lot of the boost is simply factored in but guesstimating here TBC still seems to have provided around a 30-40% boost in Classic player activity. In fact TBC raid activity seems to have peaked in September 2021 around 3 months after launch.
There is zero evidence in Blizzards graph that there was any surge of people resubbing for 9.1 the resolution of it isnt even good enough to discern between TBC Classic and 9.1 there because they only launched around 4 Weeks apart.
But if we use some common sense here for a second: We have one of the most disliked patches in modern wow history and Blizzards comments about that + the fact that 9.1.5 is considered the "ripcord" patch versus a Classic expansion that showed clear signs of growth even months post launch.
The only logical conclusion that isnt driven by some weird desire to justify an expansion that Blizzard has all but admitted was a failure is that Classic absorbed a huge chunk of the subscriber bleed and that without it SL would've dipped probably a couple million lower.