Tenacity has been buffed

[Absurdity]

Tenacity could give you 100% defence and 100% more hp. And 0.1% damage increase

People would still choose det if it gave 0.2% damage increase

Pretty much this. They can buff Tenacity as much as they want, we do not need the defense, and I honestly doubt Dawntrail will change that. So as long as Direct Hit/Det/Crit provide more damage it will stay a dead substat.
And the main problem is that the stat from it's inception was basically a jack of all trades, master of none. It does everything a little bit but exactly because of that 3-way split it is not allowed to do anything particularly well.