Does it guarantee a banana republic? Is the Junta not the cause of this instability? No, these are genuine revolutionary movements.
I am not saying that movements are not legitimate just because they attract US support. Rather that US support means they can serve US interests (which is destabilizing China). Furthermore they are not united movements, but many different ones.
Does the fall of the Junta guarantee a banana republic? The answer is 99% yes (IMO)
The NUG is a government in name only, in a country with such weak institutions already, it would be an uphill battle to establish an effective governing apparatus. This is only complicated by the ethnic militias who are allied now to oppose the junta, but who can say where they stand once the common enemy is gone?
The Junta is only the cause of the instability between itself and the NUG/PDF. These ethnic tensions existed in British colonial times, and even before then too.
I don't think China is married to the junta being in power, it was working with Aung San Suu Kyi closely before, despite her early western backing. However, based on a purely technical analysis, a bad government is better than chaos.