The point is you do spend about 80% of your income on infra, it's not like you buy nukes everyday. Or tech by those means. And to purchase improvements you have to buy INFRA. That's pretty much the only way older nations grow, other than wonders once a month or a very constrained amount of tech with at max 6 FA slots. And at the level nations at around 4999 infra can make a fair amount of money per day, you can purchase an IS fairly quickly. It's more for short term rather than long term especially for the 5000 infra jump