You are relying ENTIRELY on "it's already claimed" as the futility. I'm suggesting re-angling for "not actually up to improving on Astartes kit across the board" or "trouble deploying due to overhead" instead of a vague "they don't have enough". Because that "they don't have enough" isn't locked-in, the Imperium's DEFINED by being a fractious clusterfuck and as such the Tau Empire could expand several fold and end up with allied Forge Worlds pretty much whenever out of utterly routine treason and incompetence lining up slightly better for them.
If they're just flat-out better and counterbalanced solely by the "futility" of the Tau in an abstract "Just So" capacity, you are relying on YET ANOTHER layer of ham-fisted statements of future events that cannot be covered without nuking the setting, like pretty much everything regarding the Tyrannids. "Presently visible" means "actually happens NOW", not "can be reasonably inferred but covering it would end the product line".
Fuck ominous forewarnings, fuck every single halfwit piece of shit writing for the Tyrannids, fuck every actively malicious jackass writing for Chaos, the setting is explicitly not a block universe with the time-travel going on and as such shit is NOT locked in and doing so to declare WHAT YOU ARE PAYING FOR doomed is comprehensively retarded in both worldbuilding a WARgame and making excuse plots for a product.