It may not keep them as far away as you’d think. Before the WotPS, Don Carlos was the favored successor of Gian Gastone of Tuscany. He had to renounce that claim (and Parma, which he already possessed) to keep Naples and Sicily, but if he never conquers the latter he’s likely to keep the Medici succession (which derails the Francis Stephen Lorraine-Tuscany swap, by the way). That leaves Tuscany-Parma as a fairly large chunk of Bourbon territory in the middle of Italy, and when the alt-WotAS comes it seems likely that the first Spanish move will be to drop off a big ol’ army at Livorno. Such a move would isolate Naples from the rest of the Habsburg realms, and unless British naval support on Austria’s behalf is prompt and effective Naples may fall relatively easily to a Spanish attack from Tuscany.
Another thing to consider is that Maria Theresa conceived of Naples as her "compensation" for Silesia - she didn't want to cede Silesia to Frederick, but if she had to, she wanted at the very least to make up for her loss in another area and preferred Naples. Britain's failure to support her in that conquest and then their failure to secure it in the peace, whatever good reasons the British may have had, destroyed their relationship. If the WotAS starts more or less as OTL and Naples is already Austrian, the Queen's desire for compensation will have to find some other outlet - maybe Parma or Tuscany, but Bavaria may be even more likely, and you might end up with Austria being much more set upon eating up Wittelsbach land to settle the balance after Silesia's loss (assuming Silesia is lost, at least - Frederick's victory is by no means a certainty).
All that said, there are a lot of possible butterflies in the few years between the two wars, and I think much depends on how exactly the Austrians manage to keep Naples/Sicily. The British joining the WotPS could do it, for instance, but that has the potential to change the WotPS (and thus the WotAS) in other, far-reaching ways.