This seems like a false expectation, as it certainly doesn't jive with history.
The only movies that have crossed that line are usually big team up Avengers movies and Spider-Man movies. Captain Marvel got there too, but mainly because it sat in between Infinity War and Endgame and it was seen something that had to be seen before endgame given it was teased in the end of IW. Black Panther also got there, but it was a phenomenon on its own.
Also, T:L&T is on pace with Ragnarok domestically, and that was viewed as a big success. Not having China\Russia in its box office is a difference of ~130mil. Add those back in to make a fair comparable and it would be a perfectly fine number around $850mil.
I agree. No way the expectation is for all of these to make over $1 billion. In fact, only 10 out of 29 MCU movies have grossed over $1 billion worldwide. Like you said, the only movies the expectations should be above that really are all Avengers movies, as well as maybe all Spider-Man ones now, even though the first didn't even clear it. Regardless if you think quality is lacking or not, I'd hardly say it is showing up in the numbers yet:
-Of the 6 Phase 4 movies so far, 2 of them have cleared $1 billion, which is right on par with the previous phases percentage-wise
-Both Spider-Man movies in Phase 4 cleared the $1 billion mark, while the first Spider-Man movie previous to Phase 4 did not
-Doctor Strange 2 nearly cleared $1 billion, and made $954 million as compared to $676,000 for Doctor Strange 1
-Thor: Love and Thunder has nearly made $600 million and is on pace to end near Thor: Ragnarok's $850 million total, yet has already cleared Thor 1's $449 million total
-I'm not sure a single Phase 4 movie has been released in the world's 2nd biggest box office of China due to political reasons verse nearly all the previous phases' movies were
-Multiple Phase 4 movies were greatly affected by the pandemic, especially Black Widow and The Eternals
Again, you can think what you will about the lack of direction for the MCU or the waning quality to some level, I'm just arguing it hasn't exactly showed up in the numbers quite yet. This could change in the future for sure, but to say we are already seeing it isn't exactly true.