That still means he loses veterans of numerous campaigns in a time when the quality of individual soldiers was very important. Any fresh replacements won't be battle hardened or supremely assured of their skill by years of fighting.
We are talking about the Phalanx and Mixed Arms right? The former defined by the unity and discipline of the group, rather than the individual? One veteran, each able to claim their own reputation and status, can train a hundred men and instill them with confidence. It isn't like they're about to all disappear. The new armies will be plenty capable, especially since they're led by the same men who created those veterans. I see little reason why Alexander should be worried about their ability. It isn't like fresh Macedonian troops post-Philip let Alexander down.
I never said it did, I just pointed out how Alexander would likely have to exert quite a bit of effort to maintain his empire in India. He may be great, but he can't be everywhere at once.
Once again, those troops will be of inferior quality to those lost. Maybe numbers will make up for that, but the veterans will sorely be missed.
That still leaves a strong empire on the extreme fringes of the empire. Just that fact alone means Alexander will have to sink significant resources on the Indian frontier, as well as him needing to be able to respond relatively quickly to crises.
Every time Alexander retreats from the West, that's another campaign that the states there can adapt to, and as the Second Punic War shows, as long as you don't lose you will eventually learn how to counter your opponent. Alexander may be a great general, but so was Hannibal and he was still defeated by Rome even after ravaging their heartland for years. Imagine how a state that can freely make use of their resources between campaigns will be able to prepare, not to mention get more time that Alexander might die in.
As for marching on India with a host of a million men, how is he going to supply them on the way there? His first retreat from India resulted in a large amounts of deaths due to marching through a desert and if he takes a longer route, he gives Chandragupta more time to consolidate. Not to mention that most troops likely raised for fighting in this hypothetical host will probably be levies and not the higher quality forces Alexander used to create his empire. Or that Alexander likely wouldn't know what to do with such a large army since his conquests were mainly done with his smaller, more elite Macedonian core. There's also the historical precedent of both Persian invasions of Greece for supposedly invincible armies being defeated by an inferior foe, and Chandragupta will definitely be closer to numerical parity with Alexander than the Greeks.
It entirely depends on whether Chandragupta considers it worth it. Alexander, like himself doesn't really have a major threat to him besides the other at this point. Is it better to unite India outside of the Indus and territories Alexander controls, or risk the invasion Skall mentions. This is Alexander the Great. He doesn't DO small wars. He does All Or Nothing. He has that reputation, he just destroyed Persia with Greece, and can levy that entire might against India. Against a successor? Sure. But to risk unending war (Which is what I'd expect of Alexander in all honesty) - A western war is a drain, but a small campaign in comparison. There is nobody there that is remotely a challenge on the scale of Persia - and so accomplished (and loyal) Generals, with new ones in tow, are more than enough to do the job. There is nothing Alexander the Great would enjoy more than answering Chandragupta directly if he invaded India. It might take some thought to adapt his strategies to war elephants more effectively - especially working with Porus as a Satrap.
(Also, really, Gedroisa? That isn't an argument against Alexanders logistical potential, since the two main hypothesis' are Punishments, or "Because Cyrus Failed".)
We have to remember that it is still debated today as to what effect Alexander had in terms of impacting the demographics of Macedonia, as he called for so many men. That doesn't suggest to me that it was always top-of-the-line recruits, but every man that wasn't otherwise needed. This was so bad that Alexander when he retired some men literally ordered them to have children, and lots of them!
As such, it suggests that Iranian troops (not from Persis proper mind), were probably already part of Alexanders army in the Indus Campaigns.
http://www.academia.edu/1492980/First_Iranian_Units_in_the_Army_of_Alexander_the_Great
Seems an interesting read. It even concludes that the force that invaded India was mostly Iranian. Which suggests that it is entirely possible to increase the size of his army without it involving 'levies'. Chances are that like Alexander and Philip, the armies would be trained and made more professional as reinforcements to support the initial campaign.
Personally, I don't see Alexander campaigning against Chandragupta, or vice versa. I also don't see Chandragupta doing so before Alexander has a chance to invade Arabia. With Arabia secure, Alexander can set up major logistics on the coast, and as long as he can secure coastal ports - can supply his army that way.
This isn't a case of immovable object vs unstoppable force. It is more a case of "who is more nuts". Alexander is a talented campaigner, but certainly the more
mental. You don't declare war on unrelenting crazy who just conquered an Empire in his youth (the latter applying to both sides).
I'm not sure either Empire could realistically survive that war.