With the infamous doctrine-swapping-for-free exploit gone we felt that the cost of switching was perhaps a bit too high now, scaring people from changing up things. To make that a fairer choice starting level of land doctrines now cost half of what they used to (so in line with other techs), making a switch still a serious tradeoff you need to think of, but not as delaying is it is now. The reason it’s not just free is at the heart of HOI4. You step into the shoes of the nation at a historical point in time, so you should need to deal with their current situation. Switching army wide doctrine is one of those things, just like division templates or low war support or stability.