Note that even if you have the private key for a specific certificate, you still cannot perform a passive MitM attack against servers that use modern TLS using perfect forward secrecy, and active MitM attacks can sometimes be detected by the web server itself. There are different techniques that have cropped up; here's an old doc page about Caddy v1, mainly because it's the one that I remembered first:
https://caddy.its-em.ma/v1/docs/mitm-detection
That said, as others have mentioned, CT logs basically foil direct man-in-the-middle attacks abusing CA certificates. The attack will work, assuming it isn't foiled by HSTS, but it will be detected. For a government surveillance program, this would obviously be a very bad outcome.
The CA system definitely gets some deserved flak for being flawed, however I've personally found myself impressed with how much practical security against attackers the web ecosystem has managed to build up. It also was probably good to get more of it done ahead of time before governments could try to abuse gaps in the system; as it stands now, if we had DoH and ESNI (edit: or, now, ECH, I suppose) deployed widely across the internet, it would probably render this entire government surveillance operation useless.