No, no, you have no idea what you're speaking about. There was a prestige to timing the first +15 in the world (and for the smaller fish - realm first +15, which was a
Feats of Strength achievement). For a large chunk of high rated M+ players. You only think it was pointless because clearly you lived in your own bubble.
1. BfA M+ was unlocked on
September 4th, 2018. The gear was capped, as usual in the heroic week.
2. World first +15 Atal'Dazar was timed on
September 9th (video
here), with ilvl 350-357 gear (and 30 sec below the timer). Next day they also
timed +15 Freehold (5min30sec below the timer).
3. Mythic raids opened on
September 11. This is also when the obtainable item level was increased beyond ilvl 355.