The replacement is far from easy: Index is just terrible. While I have no illusions that if Delver is the best thing you can do people won't play it, it's a very bad card in most situations. It is, however, better than Ponder if all you want to do is stack your deck. But stacking the top 5 cards of your deck isn't an effect that's usually worth a card.
Delver also loses access to the variety of spell effects they get out of Phyrexian Mana spells. Specifically losing Dismember and Gut Shot is no small loss. Gut Shot was important for Delver to fight decks reliant on mana creatures and gave Delver something useful to do against Thalia. Dismember gave Delver a way to actually kill larger creatures like Restoration Angel that are now much more of a problem. And these effects went extremely far with access to Snapcaster Mage.
The deck might still exist, but it will be much weaker than it was before unless Return to Ravnica gives it a gift.