For reference, my detailer buddy had a lly with 350k on it. Never touched the trans fluid or filters. The most he did was check the level.
At 300 K I went ahead and dropped the pan and change the internal filter and external filter. Drove that truck around for him for 100 miles and then did another drain fill on it not one issue with the trans since.
The internal design of the Allison makes it extremely resilient against poor maintenance compared to many other transmissions. Now that doesn’t mean that every trans is gonna last 500,000 miles without maintenance. That external filter is constantly sending fresh fluid into the valve body after it’s already gone through the internal filter. The internal filter is there to filter for the rest of the trans.
There are very few transmissions we will touch at the shop in situations like yours. Alison is one that we will.