How so? You roll versus a flat value. As die pools increase it starts more and more approaching that anyway. What would be the problem with it other than its not what you're used to?
Because that's not actually how the math works. D6 shares with all dice pool systems that its math is not intuitive - one of the reasons I've decided I don't generally like dice pool systems. (D6 is the one left I will actually run, precisely because it uses both opposed rolls and static DCs in combination and notably doesn't count number of successes.) For the size of dice pools used in D6, the standard deviations are unintuitively large. D6 is actually utilizing this to ensure almost all characters have a non-trivial chance of success against almost all other characters. I wrote a program to visualize what the real math is and it's rather surprising.
But if you move to a static difficulty the math changes wildly. Think about it this way. What is the chance someone with 4D beats someone with 8D? Small but possible, despite 8D averaging 28. Now what is the chance someone with 4D rolls above a 25? The variation in the difficulty turns out to be useful in a non-trivial number of cases. If you don't vary the difficulty then the game changes radically.
(Notice that D20 handles this problem in a different way with a "20 always hits rule" where combat rolls with a 20 always work, but non-combat rolls with a 20 don't necessarily do so. It's a solution to the same problem.)
I mean, a lot about the math of Hyperspace D6 changes the game radically. Like for example, cover or concealment adds static difficulty of +5 means that it's very hard and generally pointless to take cover. In base D6 cover or concealment can often add 3d6 or 4d6 to the DC. +5 isn't even as effective as say half-cover. But then, there is that problem with fixed DC's again. If they made cover more effective, the lack of difficulty variation would probably be a near hard counter to most NPCs.
But my guess is that in play none of the math actually matters that much anyway, since there are no permanent consequences to combat and probably you have as a process of a play a lot of fail forward assumed by the GM to hard steer the game into story. That's how the author can get away with boons and burdens being so wildly unbalanced, and how the author can get away with going to a hit point system with so few hit points and so forth.
