IT01 has almost no driver flex actually - all Dynamic Driver IEMs need to be inserted slowly, except for fully open designs - IT01 is one of the happiest ones, almost no flex.
It is true that is you insert them quickly, they flex a bit, but inserting a BA quickly can cause serious pain, and I had at least a few IEMs that have considerably more flex.
About flex and durability, I don't think it matters - ie800 has driver flex, but it held up well to the test of time
(Sorry for going off topic)
But IE800 had no driver flex for me and they are designed to have none - one of the rear vents is connected to the front chamber via a tunnel of sorts, it acts as a front vent which removes pressure when inserting them. Could it be that you were covering that vent with a finger when inserting them and that caused flex?