use of the infinitive after 'Il ne manquerait plus que tout s'effondre'

Question

Why is the infinitive used in this sentence after 'tout'?  What does this sentence mean exactly in English?

Cécile Janty-Davies · Accepted Answer

Bonjour Sarah, 
1. 's'effondre' isn't the verb's infinitive, which would be 's'effondrer'. It is a subjunctive present that follows the impersonal expression - Il ne manquerait plus que ....
2. As Chris has shown it is quite difficult to translate literally.
This expression is used sarcastically or ironically to express exasperation or frustration when things are already going badly, and the speaker is imagining an even worse scenario. It's similar to the English expressions:
'That's all we need to happen now!'
'The only thing left is for everything to fall apart!
'As if things weren't bad enough already!
 
The lovely expression -
'Il ne manquerait plus que ça !' ( often said in everyday speech as 'manquerait pu qu'ça !' 
which can also be used in the imparfait to express something that has already happened - 
Il manquait plus que ça !
is a common French phrase that fits many situations to express exasperation or frustration, similar to
'That's all we need!' (or needed )
'As if things weren't bad enough!' or
'Things can't possibly get any worse... or can they?'
Bonne Continuation !

Chris Wirt · Answer

Il ne manquerait plus que tout s'effondre. -- (lit.) It would not lack anything, so that eveverything falls apart. Or, more meaningfully, "The only thing left would be for everything to collapse.To parse that sentence, note that the group tout s'effondre belongs together. The verb group s'effondre is subjunctive and refers to tout.

Alan Graham · Answer

It's not an infinitive - s'effondre is the 3rd person singular of the verb s'effondrer. The subject is tout.