Lol its not like that bro it has nothing to do with falling back to 2G/3G FDD+TDD was introduced in 2015 or something idk before that there was only FDD+FDD and TDD+TDD so phones required FDD band as base to get CA, While if they latch to TDD band it can't give CA, Nowerdays TDD+FDD is possible i think, have read something about it not sure, so even it latches to TDD band it can combine with FDD, so currently device support is very rare for that kind of CA