This actually proves you wrong. If you can survive that Crunch and Will-o-Wisp (which you just proved is a certainty), Sitrus Berry will activate and drive you up to an average of 46% HP after accounting for Sandstorm damage. A commanding majority of the time, you'll be able to survive the second hit and use Leech Seed and a combo of Protect and Harvest to get you back above KO range. Even in the worst-case scenario that Crunch does maximum damage, you're looking at Sitrus Berry getting you back to 39% HP after sandstorm damage, with a 50% chance of Harvesting a second Sitrus Berry at the end of the turn to pull you back up to 58% HP after Sandstorm damage. And even in the event that you don't get that Berry, you can use Protect again the next turn for another 50% chance to Harvest your Sitrus, which would bring you up to 52% HP, at which point you can use Leech Seed/Protect in tandem with Harvest to ensure that you're safe from being KO'd. So even in the very unlikely scenario that Crunch does within 2% of maximum damage two times in a row (highly unlikely), you still have a 75% percent chance of Sitrus keeping your HP high enough to handle Tyranitar with ease. Overall, the chances of me surviving a Tyranitar encounter are over 95%, which I would say falls well short of being "maimed". Even in the event that Crunch hits that very small chance of lowering my defense on the first try, having Protect gives me a 75% chance to switch out with over half of my HP remaining. I don't even need to tell you how pitiful Pursuit would fare against this Trevenant, not to mention that I have encountered exactly zero Tyranitars that run both Crunch and Pursuit. Additionally, I can't even recall the last time I ran into a Choice Banded Tyranitar, and the rarity of it is a risk I'm willing to take in order to know I can survive a Shadow Claw or a Swords Danced Earthquake from Excadrill.
What I'm saying I guess is that you can run damage calculations until you're blue in the face, but if you don't take the game mechanics into account, you're not proving anything. In this case, you've actually proven yourself wrong. In your defense, it seems as though you didn't consider that I might be running a Sitrus/Protect set, so maybe just don't go through all that effort next time unless you've checked all the facts of the set.