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.