This is what people say, but I don't think that's true.
Saturday Morning just used to be the day the NEW cartoons aired. Re-run blocks happened all the time. To the point where multiple cartoon channels formed in the 90's just made of re-runs. CN today is mostly re-runs.
Having all of the new cartoons airing on the same day was extremely convenient for companies because it was all of the young eyes viewing TV at the same time when they all had the day off. The excuse that it was made obsolete is exactly that, an excuse.
The real reason Saturday Morning blocks died is because cartoons stopped being profitable to create and serialized western animation is in the shitter. Very few cartoons can make a set schedule, and even 3D animated shows have problems making weekly deadlines to even survive on places that are dedicated to showing exactly that kind of content like CN.
There's a reason people joke about steven bombs being worthless; if they could keep a consistent schedule, then they would show those kinds of shows every Saturday as they did previously. However, that's just not profitable anymore and most of the animation companies that used to be able to do that either closed down to transitioned to things that are far more profitable like movies.
But no keep telling yourself that the internet made it obsolete even in an era like the 00's where people had internet that was fast enough to watch full-length cartoons on. Like I'm sure most people here did at the time.