Way too convoluted to put something this minor behind an upgrade.
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but for real, the vet 3 damage reduction thats designed to make grens not evaporate instantly in the late game should be coupled with the mp reduction with tech not vet. it could still be unlocked at t4 but we need to stop pretnending that t4 is an alternative to t3 and make it a premium tier once again. treat it somewhat like hammer/anvil where you get it for late game buffs. if it provides more than just units then it can actually be worth getting if the player plans on t3 units.
i have no idea how something as cut and dried as a battle phase teching with 7 levels of tech could be mishandled so poorly but here we are. each building should offer something to make them attractive despite the units within, battle phase escalations should improve the units in the tier before to promote fully teching and FEELING like a damn escalation of power.
imagine the old ost tech costs, but teching battlephase 3 grants stugs and p4 skirts as a global passive and the ostwind better AA? well now you are already part way to t4 and even if youll never afford that panther, building the building itself will make your infantry tougher and stem some bleed... well now the game HAS dragged on and you lost your vetted p4, you have been floating a bit of fuel, you might as well bring out the panther because they are pumping out tanks like a soviet factory. already have enough AT? that brumbar will round out your army and force the enemy to concentrate assets to keep their infantry alive.
making each step attractive beyond the unit makes teching up rewarded and more of a struggle to decide if its worth missing out on bonuses to race out faster units.