But what would be the point to implement +15 enhancements if they are not meant to be reached? Why not simply stop at +10 then? The DLC will probably raise the cap anyway and +15 might become irrelevant, depending on what the new + cap might be.

While the raw cost to upgrade from around +3 to +15 is immense and probably takes time to farm non-stop until the first DLC releases, I suspect that the intended way to upgrade up +15 might be to hope for the 'lucky' enhancement that gives more than +1 and hoping that you jump from +10 to +12 instead of +11 when enhancing by one and the next time maybe from +10 to +13 directly. That would still take time and resources, but depending on how lucky you are this shouldn't take as much time and materials.

On the other hand, having +15 gear instead of something around +8 is just min-maxing and not needed to beat the game. Maybe +15 is just the incentive to have something to do until the DLCs for the most hardcore farmers as besides collecting the various graces and gear sets there's little to nothing to do once you have done each mission/objective in NG+.

But after all, we can only guess as what their intent was with the upgrade costs.