I just want to quote this as I think it's a really good point. And get those multipliers together on defenses and DPS and we have the power creep problems we have now. Worse, the multiplicative abilities (+1 crit range, +10%HP, doubleshot, etc), are so much stronger at cap than the additive ones (+1damage, +10HP, etc), that they make the latter almost irrelevant at cap, and because they're so much stronger it's the multiplicative things being added to all the classes in each pass to make them catch up, and because the older abilities unique to each class are so much weaker than the new +1crit range etc, each class starts to feel more the same...
Multi-classing sometimes lets players get more stacking multiplicative effects for exponential power, but these days most classes have just as much exponential power increase in staying pure, so no, multi-classing isn't contributing to power creep, and really no need to constrain it further.
I also agree on your solution to that problem, increase cost to get those high multipliers, though that's easier said than done. Maybe in 3 years when they finish the current overpowering class pass on enhancements they can start the rebalancing that way (sigh).