Trip damage doesn't scale for most people. Some classes can make the critical success crit damage better (barbarian, maybe monk) but in general it is a side benefit. If you want to do damage, you strike rather than trip. You trip to apply the prone condition to enemies, and if you do it well you also do damage. Seems like a sweet deal to me, but your mileage may vary.
Something like the barbarian's Brutal Bully improves the damage for all trips, and something like the Fighter's Improved Knockdown does not actually key off the Trip action.
Perhaps, in some ways, the Trip action was meant to be less and less attractive as the levels go by. I do not know.
Gets far, far more attractive with age from where I sit. The damage is nothing. -2 AC and Attack rolls that targets a reflex DC, that anybody in the game can become legendary with, and that the enemy spends a whole action if they want to negate it is a pretty sweet deal. It also knocks people out of the sky if you have flight to pursue them. That you have a slight chance of also doing 1d6 bludgeoning is just gravy, prone is the main course.
I am not contesting that prone is a good condition to inflict. It is the critical success that has me wondering. How exactly does that "get far, far more attractive with age"? Inflicting prone is an automatically-scaling benefit, but it does not necessarily get inherently stronger as the levels go by.
The damage doesn't, apologies if it seems like I was trying to say the damage does get better with age. The action as a whole does. At level 1 the tricks available to monsters you expect to fight are generally less devastating, so removing 1 action or inflicting a -2 attack/AC is less valuable than at level 15. The d6 damage is nothing but gravy on top of a relevant effect.