He was overextended, he admitted that Porus was a strong opponent and his army mutinied or nearly mutinied because they were about to face the Nanda Empire if they advanced further beyond the Hydaspes which is why he turned back. Sure, Alexander conquered fortified cities, but had he faced the Nanda Empire, he most likely would have died.
Which he isn't doing because not only is his army exhausted, he would be facing an empire far stronger than the outskirts cities he captured, for as impressive as those conquests were, he almost died doing so (receiving the famous arrow that almost killed him in modern day Multan) and these confrontations were the reason why his army nearly mutinied and forced him to go back. They weren't easy by any means and he was about to confront an even more powerful centralised state that was quite comparable to the Achaemenids at least militarily and in terms of infrastructure. So yes, it really is implausible even if Alexander decides to use Asians and send back his Greek army.