Not sure if true... but I can see why the WoW devs wouldn't want to take the risk.
I can say as a long time ESO player, I saw it start out the same way as WoW with the constant power upgrades, then they capped it and left it that way ever since. It's been much better and has allowed for other innovations like the item stickerbook where you basically can collect every item at end game power level. Their situation was much different, they had a smaller playerbase and the game wasn't in a good state anyway.
But I think about how much that would bring to WoW. Every tier set in the game is now at max level and can be selected as a gameplay choice. Every trinket can be relevant. Shit I'd like to use Soul Capacitor again on my rogue, that was a really unique gameplay effect that was just rendered obsolete. /shrug