I think maxing out the doctrine tree should not be possible before '44 or even '45.
If this were the case it would be impossible for France to switch doctrines, given the penalty they have for land doctrine research. It would also be damn hard for anyone else switch doctrines, because putting yourself 2 or 3 tech behind in land doctrines even right at the start will mean not having the tree finished before the game ends, and not getting the "best" bonuses at the bottom of the tree.
It look more like a balance problem then anything else as they forget about so many other technologies to be able to research doctrines constantly.
Johan commented a couple of times in the stream that doctrine bonuses are so good they are a no brainer and so expect these to be modified somehow; either their bonuses reduced or their cost increased or both or who knows maybe some other solution entirely. but I'd be surprised if the land doctrine tree remain unaltered.