Yet another vote for Tumble being fine as is.
I think the problem people percieve is they crunch the numbers and realize that at level 12 or so, tumbling past opponents becomes trivial, and at 15-18 tumbling through opponents becomes trivial.
Then they stop, don't think any further, and yell "OMG WTF BORKEN!!!!1eleven"
So. What.
At lower levels, tumbling is far from trivial to succeed. At mid-levels and higher, it's generally trivial if the tumbler does succeed! In most cases, the tumbler moves past foes to get at the back ranks (usually the BBEG) and often can make a single attack (if even that). Then the Fighter full-attacks the hell out of the (usually low-HP) tumbler, or the wizard 5' steps back and nukes/controls him or ports away. And often the (again, usually fragile) tumbler is stuck in the middle of a bunch of enemies, who are probably pretty pissed at him.
*shrug*
Look beyond the auto-success numbers. So a skilled tumbler can easily bypass many enemies to get to the back line. So what?