At 4'th level character who has a 16 DEX (+3) and mobility (+4) and 7 tumble ranks (+7) has a +14 tumble bonus already, for automatic success against the first opponent. What's the point of having a tumble roll when automatic success is so easy to reach?
At 10'th level a rogue with an 20 DEX and +4 Dex nimble gloves (or whatever), mobility, and 13 tumble ranks, has a +24 tumble check. The tumble DC for moving past opponents has become rather completely pointless.
There is a problem when player / opponent CRs become too far apart.
Having fixed DCs for what are in essence are opposed checks seems to be an odd design.
It's not really an opposed check. It's a check to determine if the tumbler can move without dropping his guard enough to allow an attack of opportunity. As such, it can easily seen to be all about him, not his target's ability to fight. That said, I would have been content if PF had included a feat or two (perhaps in the combat reflexes chain) to specifically raise the DC of tumble checks around that opponent.
But let's look a little more at your estimated values. Mobility does not add to tumble checks. It adds to the AC of a character suffering an AoO from movement only. So that 4th level character's ability has just dropped to +10. Still an easy check, right? That's why the additional modifiers on the DC become interesting. He may be able to move without an AoO on a flat, clear floor and at half speed - netting him, on average, 2 squares more than his 5' step - not exactly ground breaking but nice and with only a 20% chance of failure. Suppose he wants to move farther, particularly if he wants to get through the threat range of a large creature to flank it with his fighter buddy? Moving half speed probably won't cut it so he wants to do a full speed tumble - which has a DC of 25. Now, it's not so easy and he's more likely to fail (70%) than succeed.
Or maybe he wants to flank the guardsman who is blocking progress in a narrow corridor. To tumble through, he needs a 25 to move half speed. Again, more likely to fail than succeed at 4th level. Make it a huge creature and he needs to full speed tumble through at DC 35.
Now suppose this is going on in a natural cavern with its typical uneven floor. Add 5 to all of those DCs. Even a mildly difficult area like gravel, slick with dew, or with undergrowth adds 2.
It's true that a high enough tumble value makes rolling the tumble check pointless (though I would prefer the term unnecessary) - this is not a bad thing. Being able to have an auto-success makes the game run smoothly. But it does require a non-negligible investment to have gotten that far. And, in my book, that's good enough.