As a 4th level warrior-type, I get 1 attack when I take the Attack action. If I have a feat which lets me take a bonus action if I take the Attack action, then the feat lets me do something MORE than if I don't have the feat. The feat makes me 'better'.
If that 4th level warrior took, say, +2 Str instead of that feat, he would also be 'better' than if he hadn't. Feats/ASIs make you 'better'.
Now the +2 Str version levels up to 5th and now gets the Extra Attack feature. Now, his Attack action lets him attack twice instead of once. This makes him 'better'; exactly 1 attack 'better'.
But the version who took the feat (the new interpretation of Shield Master) is not made 'better' when levelling up and getting Extra Attack! 'Better' would be getting one more attack AND the bonus action, every time! But now, if the situation is that you want to shield bash the adjacent foe off a cliff and then move to another foe 30 feet away and use your 2nd attack, you can't! You have to use EITHER both attacks and then shove, and lose the option to move to his mate and attack him. That attack doing pointless damage anyway because the fall from the cliff will kill him anyway. OR you attack, shove (just like you could do without Extra Attack), but you would lose your extra attack!
This is not 'better'! It should be 'better'. It was 'better' with the 'either order' interpretation, it was less good but still 'better' if you can take your bonus action after one attack, but it is quantifiably not 'better' if you have to choose between the shield bash or extra attack when you should be able to do both.
Just to illustrate: imagine that the version who took +2 Str instead of the feat had to choose whether to take 2 attacks OR add +2 Str, but not both in the same round. That would be obviously absurd. Well, this ruling is also absurd, but the absurdity is less obvious.