This is patently false. Your relative power did not get weaker. If a mob was level 50 and you hit it as level 50, that same level 50 mob would be weaker when you came to hit it again at level 53 for various reasons. Now if you level up and go to that same mob, it too has leveled to 53 all the while your own relative power to it has grown weaker.
The only way you can say that "It's been this way since the game started" would be that you consider game having started in Legion.
This obnoxious all-encompassing scaling is one of the main things I've critiqued since start of the Legion as it alters the dynamic of leveling entirely and undermines the sense of progression you might otherwise have, eg. if I wanna push harder higher level quests I could, if I felt weaker then I'd take on lower level quests and mobs until passive benefits from leveling up overcomes the perceived difficulty.
This is simply not true anymore at this point and time and it'd be very easy to fix too by having zones lock to a certain level range after you select them and then only scale world quests up once you get to them.