[XorNot]

This is missing the point though: it's not "optional" per se - it's that naively, if we simply go "what if FTL" then yeah, based on current physics we have a problem.

But the logic doesn't then mean "can't FTL" - it's still "can't have paradoxes". An FTL system, theoretically incapable of creating paradoxes, has no such problems.

An FTL system which prevents you from going faster then light in directions which allow violating causality would be totally fine, provided this was a theoretical restriction: i.e. your FTL engine just plain can't thrust in causality violating vectors, because it encounters some temporal restriction field or similar.

So the problem isn't "FTL is a causality violation", it's FTL without a mechanism to prevent causality violation is probably impossible.

The sci-fi conceit of it being difficult to plot efficient paths through hyperspace or whatever might well be quite a real thing, and simply related to charting causality-allowed courses through FTL-space.