Yeah I agree, Italy being CP would be pretty decisive. That said, re-rolling every dice from a PoD of Italy honouring the alliance still leaves a fair amount of wiggle room, enough for maybe a 5-10% chance of the Entente pulling through. If the Ottomans decide they'd rather remain neutral than ally with Italy Britain has a lot of manpower freed up relative to OTL. Also, Britain ended up sending troops to Libya and Somalia iOTL as well, so having to fight the Italian colonies doesn't change the balance sheet too much.
Consider that Italian forces in Somalia at that point were ludicrously small and the place is exposed to British holdings on every side. That theatre in particular is so lopsided that it's not even funny.
Libya was, by 1915, in the situation where local forces seemed close to expelling the Italians
on they own, with some paltry covert Ottoman support. The British need just to make a deal with the rebel leadership and use Malta to intercept Italian reinforcement there, though if the Ottomans are CP as well that may create some very strange diplomacy: Italy migh cut a deal with the Libyans instead, which
would create a potential threat to Egypt... but it is the same threat it faced IOTL, as the Libyans were loosely CP-aligned and coordinated with the Ottomans (and, theoretically, Darfur) a joint invasion of Egypt that was repulsed easily. Even with the Italians on side, they would not add much to that doomed plan, simply because they paltry and scattered forces in Libya are cut off.
Big deal would be if Italy is able to seize Malta early upon entering the war, but I highly doubt this is possible or can even be attemped.
So, the only colonial theatre where CP Italy is even meaningful would be Eritrea, where local forces under Italian command and decent defensive terrain might make the British having to redirect some units from elsewhere (the place is also cut off, so the final outcome is beyond question: Italy loses it, probably in months).
Where Italians really matter is in European theatres, particularly regarding Austria's manpower thus freed, and French troops conversely tied down into a likely static Alpine front.
Also, with Italy onside the Ottoman position looks more secure. It is not guaranteed that the Arabs rebel, and if Gallipoli does not happen, the OE has a lot more troops to deploy in the Arab provinces.
I agree that Entente victory is still
not impossible, especially if the war lasts long enough for the US to enter and change the strategic picture again, but that in itself is unlikely.