What if your "good enough" isn't as good as, or is only just as good as the enemy's? Tank, Fighter, Warship, Artillery? We're a small country in terms of population. Our potential enemies (China, Russia, Iran, North Korea, etc.) have larger populations/militaries so we will not overmatch them in quantity. Should we lose our qualitative advantage as well?
I get the whole "quantity has a quality all its own" concept but it can only work if you have a relative quantity advantage over your adversary. It's fine to say that for the price of 80 x Leopard 2A7's we could have 300 x Leopard 1's. That's certainly a quantitative advantage over what we have now, but in relation to the number of equally effective tanks that our enemies have (Russia - 12,267, North Korea - 6,000, China 5,750, Iran - 2,842) we are still greatly outnumbered. Same for every other major piece of military hardware we have.
We're not going to match our enemies in quantity. If we want to give our soldiers/sailors/air-humans a chance to defeat our enemies (and survive in a conflict) then we owe it to them to give them the greatest qualitative advantage we have.
Of course there are limits to the concept. If you were to buy the absolutely latest, greatest cutting edge technology for every capability you'd use up your budget and have a military too small to be effective. But to my mind we should strive to field equipment that on average exceeds the quality of the equipment being fielded by our foes.