That is a good point.
I hope every class chooses its subclass at level 1.
Why should that be?
I think we will see how the cleric does it.
I can see many impelentations that work well with cleric subclasses relegated to level 3.
Level 1: chose domain/deity.
Level 2: get appropriate channel divinity.
Level 3: get a proper subclass that makes your cleric really differentiate between each other.
Same for wizards.
Level 1: chose specialization
Level 2: get appropriate power
Level 3: get peoper subclass like arcanist, war wizard etc.
Or something like that.