The cost of base damage skills effectively DOES increase as you buy more. It just increases in an invisible way.
It takes 15 build to buy a prof whether you are level 1 or level 20. But, at level 2, it takes two games to gain that 15 build. At level 20, it takes approximately 20 games to gain that 15 build. That is an increased cost since you gain the exact same benefit, but need to play 10x as many games to get it. For all the inefficiencies in our system, that part is actually quite slick.
In general, my suggestion for decreasing damage bloat is to add more cool skill options, make current skill options more competitive, and decrease the pre-reqs of some skill options. If you are spending more build on skills other than profs, then damage decreases. I know plenty of players who would happily shed a few profs for more Parries or Stun Limbs. Or imagine if there was a physical skill that let a single weapon blow (including prepare to die blows) affect an opponent even if you only hit a shield or weapon. Players would spend build on that. More viable options decrease damage bloat. Always has, always will (if there is a rogue in the game that doesn't have max Dodges, I've yet to meet them).
-MS