Formula for damage taken?

[MythToken]

But all stats will be growing. Along with enemies getting stronger too, so in reality foresight is still only going to give you 20% defense, it won't become op because of us getting higher ilvl.

It doesn't matter if other stats grow. It's based on our personal Defense rating, and if Defense grows higher then amount we get from protect will also grow. Which in turn adds to the amount of Foresight.
It can become more effective than it is currently.

The % might be the same, but that does not mean it will not mitigate more damage.
Since it is not 20% less damage taken, it is 20% more of a stat.

20% of 100 = 20 (~1% damage reduction)
20% of 1000 = 200 (~10% damage reduction)

*keep in mind the above is just an example and that Menae's explanation of damage reduction cooldowns being multiplicative still applies.