I'm not sure a French victory would be a total disaster for the allies. OTL Eugene deferated the French in northern Italy and secured Milan by 1707. Austria then made an agreement with France that allowed the French to withdraw unmolested from Italy so long as the troops wouldn't fight there again. So France was able to pull out the armies it had bottled up in Naples and send the promptly to Spain where Berwick had just driven Charles and the allies from Madrid. The deal secured Spanish Italy for Austria but greatly annoyed the allies who saw it as a unilateral move by Austria to cut a separate deal with France. It also diverted Austrian arms south into Naples instead of attacking France.
But a lot depends on how badly the French defeat Eugene. If they capture Turin before he can relieve it and then fend off his attack he may be able to withdraw in good order. The French still hold Milan and Modena and maybe Mantova but they're facing a large Austrian army across the Adige. They've still been defeated at Ramilles but have countered most of Marlborough's successive attacks. Critically, Spain may play out differently in the following year.
In 1707 the allies, having withdrawn from Madrid, still held most of Aragon, Valencia and Catalonia. There was great debate on the allied side as to whether they should adopt a defensive strategy or march once again upon Madrid. In the end the decided that Charles and his Austrian forces would sit defensively in Catalonia while the British would march forth from Valencia in a pincer with an Anglo-Portuguese force moving in from the west. This ultimately lead to the catastrophic allied defeat at Almanza at the hands of the Duke of Berwick. But with the allied defeat in Italy perhaps the allies opt for a defensive strategy. With French army still in Italy Berwick lacks the reinforcements he got OTL. And previous French attempts to take Catalonia in the 1706 had ended in disaster for the French. The fact was Catalonia was as pro-Habsburg as Castile was pro-Bourbon. So just as the allies found it extremely difficult to hold Madrid the Bourbons found Barcelona incredibly difficult to take. So the Spanish campaign may consist of a series of failed French forays into Aragon and Valencia.
Thus 1707 is mostly a stalemate but Austria is somewhat chastened and more dependent upon the support of Britain in Italy than OTL. Eugene's Italian army is made up largely of Germans bought and paid for by Britain. Joseph is going to be under even more pressure to come to terms with the Hungarians with French forces at the Adige.
So going into 1708 the allies may have a slightly better time in Spain if they still control Aragon and Valencia, rather than having been reduced to Barcelona and its environs. If Eugene regroups effectively he holds the line at the Adige. Perhaps he can gain a few victories, taking Mantova would be helpful to put the French on defensive at Milan. If he still has the bulk of his army after Turin he now has short supply lines and has shown an ability to outflank the French in the Po valley. There is also no reason to assume that Marlborough can't continue his offensive in the north. Maybe not Oudenarde specifically but a victory of a smaller scale is still possible. Then Europe faces the devastating winter of 1708-1709.
So the Bourbons still hold much of Spanish Italy but the allies hold half of Spain. Things in the Netherlands are similar to OTL, though the French position may be stronger. The allies maybe more inclined to entertain proposals for a peace settlement but the same obstacles exist as OTL, namely both sides want Spain. Philip is secure in Madrid and Castile and Charles is secure in Barcelona and Aragon. So the war probably continues at least through the 1709 campaign. With Eugene tied down in Northern Italy Malplaquet isn't going to happen and without an major allied victory the campaign in the Netherlands will just be a long bloody stalemate.
So with the allies weaker in the Netherlands and Italy but stronger in Spain, if only marginally, then a peace becomes possible. OTL Louis since 1706-07 had been willing to abandon Philip if he could retain France's borders in the north and reserve southern Italy for him. Now a French army holds Naples and Sicily, but the allies hold half of Spain. So the deal is simple, France cuts ties with Philip, but the French army remains in Naples and Sicily. Max Emmanuel is restored to Bavaria while Victor Amadeus is restored to Savoy. The allies are responsible for removing Philip from Spain but with a stronger starting position its possible. France gets to keep Tournai and the fortifications and Dunkirk so its marginally stronger in the north. If Philip fights on despite his grandfather's wishes then I could see a scenario where he is exiled to France after his defeat in Spain and instead his son Louis is set up in Naples under some kind of French regency. The big question mark would be the fate of Milan, probably depends on whether Eugene can take it before the war ends.