You don't understand how tenacity and potency work.
The chance to resist a debuff is found by subtracting the attacking toon's potency from the defending toon's tenacity. That is then the chance to resist
However there is a hard floor of 15% chance to resist any debuff.
Therefore taking your Rex as an example with 80% tenacity. The chance he has to resist a debuff will depend on the attacking toon's potency and will be
0% potency 80%
50% pot. 30%
65% pot. 15%
80% pot. 15%
1m% pot. 15%
So you are right about one thing. Tenacity is a pretty underwhelming stat and you need it to be more than 15% higher than the opponent's potency to get any benefit from it.