Only reason I can think of is that if someone worse was threatening them, which in this case would probably be France. You'd basically need a Holy Roman Empire that was strong enough to discourage or fend of any French incursions yet decentralised enough to not impinge on the Dutch successor-states sovereignty too much.
Edit: And Tyr gets in before me.
So, having Low Countries turing back to Habsburgh domination would be like rabbits stopping to run and gently placing themselves just before the hunter's gun apologizing to not being able to cook themselves properly.
Not mentioning the St-Barthelemyfest that these provinces would host at short term (hey, if they are loonies enough to submit themselves to Austria and Spain, they wouldn't be able to stop their intervention), states having formed alliance with Netherlands would probably estimate (at least fearing an imperial takeover of northern trade ways and threatening of what would remain of netherland's fleet and power) an intervention to be necessary if not vital.
Military I guess. The HRE could be pretty militarily strong.
I suppose if its a question of super centralised and strict ruling France or the very feudal and lax HRE they might prefer to go with the light touch of the empire.
A bit ASB though.
Well in 1648 the status of protestant states in the HRE was recognized with the Cuius regio, euius religio, which after the peace of Westphalia was extended to Calvinists too. The Orange-Nassau and Nassau-Dillenburg would be inclined to improve the relations with protestant rulers and states in the HRE like Saxony, Brandenburg and Hesse. Returning to the HRE would require some threat, which most likely would be France.
And contrary to LSCatalina the HRE wasn't an as powerful weapon what Austria and Spain would have wanted it to be. Yes, there is the example of the Great Turkish War, but at the same time there are various examples of France expanding into territories of the HRE (including Habsburg Lands).
Furthermore there were two imperial armies: the army of the Empire (Reichsarmee) and the army of the Emperor (Kaiserliche Armee); the former could only be deployed with the permission of the Imperial Diet (and consisted of pre-determined contributions of each imperial circle).