Conventionally, balance of power theory suggests the opposite, that ASEAN would rather band together more closely to counter both China and the Western alliance in the region. If I remember correctly, Waltz himself wrote that balance of power theory is only really apparent and applicable in a multipolar world order.
There are many lenses through which to analyze international relations, but one of the fundamentals of this field is that it is NOT an exact science and it is not possible to be predictive in any way, shape, or form. Analysts have spent the better part of the last century trying and failing.
ASEAN will do what it must to ensure its survival. Its actions will not necessarily be rational. Countries like Indonesia may buy Chinese equipment to bolster its own military security or it may decide the best security is to just not buy anything and stay out of attention. Maybe they would prefer to focus on economic rather than military security as a way to "balance" the changing dynamics of the region. And then you have to consider the people in charge, and their personal motivations for this decision. If they are corrupt, how corrupt?
The questions are few but the answers are infinite and there is no conclusive answer to any of them.
Source: Have a degree in IR.
Balance of power analyses are the best first approximation to any geopolitical analysis - that some apply it fallaciously is not to the theory's demerit. Your suggestion that the theory "predicts" that ASEAN would band together is one such fallacy: Namely, ignoring the costs of unifying (which necessarily entails abandoning some of your interests) in relation to the benefits gained by it (not much, since ASEAN doesn't amount to a hill of beans geopolitically even if it were perfectly united - so the gain to each member is minimal). Incidentally, it's also why Latin America doesn't band together against the US despite historical grievances. In general, the "complication" of unifying
n independent entities scales quadratically with
n. Properly applying balance of power analyses would take these factors into account.
To put it baldly, the failures of IR are not due to any inherent limitation in reasoning about countries; it's that this analysis is done by Americans. Their deep indoctrination and cultural idiosyncrasies irredeemably bias and damage any attempt at geopolitical analysis they make (with exceptions, of course).
There are further factors we can add to the picture in Southeast Asia beyond the obviously changing balance of power in China's favour:
First, and most importantly, China is the resident power in East and Southeast Asia. China is forever. In contrast, America is half a world away, and decisionmakers in SEA know full well that at any moment America can decide it's had enough and go home.
Secondly, ASEAN countries have had relations with China for millennia, much of which China was
far more powerful relative to them than it is even today. Despite that power, the "Philippines" isn't named after a Chinese emperor Philip. It wasn't China that spread Catholicism and destroyed their cultures. They can easily extrapolate that they'll live very comfortably in a world where China is dominant so long as they don't cross a few very reasonable red lines.
So as not to make my analysis completely one-sided, I'll raise a point in the US's favour: its Freedom & Democracy load finds a willing audience in SEA, especially among the youth (young, dumb, you know how the rest goes). I don't think that has enough of an effect to change the fundamental trajectory, but it will undoubtedly introduce some friction.
