It's a utility helicopter not substitute to attack helicopter.
If the deal is really cancel then Pakistan have no better choice then go for Z-10 attack helicopter to supplement T-129 .
And who says Viper are not coming there is no official word on it. There is no contract for placing them in storage by US department of defence. Visit the site of DOD USA and show me any new storage contract.
Here the delivery is in August 2018.
Bell Helicopter Textron Inc., Fort Worth, Texas, is being awarded a $581,113,421 modification to definitize a previously awarded advance acquisition contract (N00019-13-C-0023) for the manufacture and delivery of 15 Lot 12 UH-1Y, 19 Lot 12 AH-1Z, one Lot 13 UH-1Y and 21 auxiliary fuel kits for the Marine Corps and government of Pakistan. The aircraft contract line items on this modification are fixed-price-incentive and the auxiliary fuel kit contract line items are firm-fixed-price. Work will be performed in Fort Worth, Texas (60 percent); and Amarillo, Texas (40 percent), and is expected
to be completed in August 2018. Fiscal 2014 and 2015 aircraft procurement (Navy) and foreign military sales funds in the amount of $581,113,421 will be obligated at time of award, none of which will expire at the end of the current fiscal year. This contract combines purchases for the Navy ($523,193,712; 90 percent) and the government of Pakistan ($57,919,709; 10 percent) under the Foreign Military Sales program. The Naval Air Systems Command, Patuxent River, Maryland, is the contracting activity.
https://www.defense.gov/News/Contracts/Contract-View/Article/614822/