Oh, come on now...don't be so rigid like that...
Way I see it, it's an alternative timeline so pretty alternative stuff could and probably would happen...
So what if he could have captured northern India?
Maybe through his own personal charisma he would have managed to convince his troops to stay onboard for a final push.
Also the unrest and dissidence among his troops and generals and especially it's significance is a bit overestimated I think. Yes, they probably were tired of the endless campaigning and since Alex was open to criticism they weren't shy letting him know how they feel. Still, above all esle, they were the king's soldiers and would follow him to the end of the world, quite literally, if that is what he commanded.
Yes, it is possible he would have to remove the heads off a couple of the main dissident figureheads among his leaders if it got down to it, and that would kind of blemish his almost spotless presence in history but...
But bottom line is discipline and morale would be restored among the ranks and they would fall in line behind the king because that's their duty and job.
Additionally, it stands to reason that after Persia, the Indian states would have been the richest places EVER witnessed by the Greeks. In many ways probably THE richest. As in gold, silver and all kinds of precious gems.
Even the northern states who weren't the most powerful, nor had any semblance of strong centralised government like southern Indian kingdoms did.
So it could be that in the face of all this wealth just waiting to fall into their laps, the enthusiasm of the host would be renewed for one final drive.
Let us not forget that, no matter how overextended his army was he still never lost a battle.
Not in India, not anywhere.
Following that pattern he could have easily defeated the forces of the various north Indian warlords to finally force them into submission.
And that brings me to my final argument.
That he wouldn't really have to "capture" Northern India. All he'd have to do is sack the richest cities to provide for the loot that would placate his rogue generals, satisfy his soldiers' hunger for wealth and enforce vassalage among the local warlords.
That would provide him with ridiculously lavish annual tributes plus it would give Alex the perfect launching base for his future conquest of lands beyond India. Not to mention a huge pool of manpower from which to draw soldiers for his future armies.
What part of what just I said doesn't sound even remotely possible?
Just because it is an alternate timeline doesn't mean it has to be fantastical. Alexander had massive amounts of personal charisma- his troops loved him, they would charge a city to protect his prostrate body.
And that wasn't enough to make them go even further than Persian armies, to go further into India. The distances involved are practically preposterous. It took an army, back then, six months to march from the Aegean coast of Asia Minor to the outer periphery of the Persian Empire. Double that, and you get the kind of reaction time that someone would have in an empire that reached through India.
The Romans, who had a massive road building system, still could not react faster than about two or three weeks to events occurring in Gaul, despite everything they did, they were still behind.
Should an attack fall upon this Empire, it would almost immediately crumble. The Empire wouldn't be able to pay, especially if it was led by someone of Alexander's caliber of spendthriftiness, for the necessary army in India.
Additionally, there is the possibility that Alexander was forced out of India by some foreign power, rather than by his own volition, giving Porus the lands he had conquered. It may be that he lost a battle there, and had to retreat.
We can't know, speaking more than 2,000 years away from his death. We do know, however, that logically speaking, an Empire big could not survive in the pre-modern era.