IA is buying the M777 lightweight howitzer for deployment along the mountainous border with China. Otherwise, there are not many takers for 39 caliber towed guns these days.
I find it inappropriate to compare M777 with M109 SPH. The latter can keep up with armour and provide indirect fire support.
Guided munitions are not restricted to M777, the M109 can fire those as well.
From:
https://www.baesystems.com/en-uk/download-en-uk/20180620191930/1434602522105.pdf
For land platforms, Vulcano can be fired from all variants of BAE Systems’ M777 and M109 howitzers and the majority of weapons for allied militaries around the world.
As the original equipment manufacturer of these major gun systems, BAE Systems and Leonardo are best suited to integrate Vulcano into these weapons.
As NATO is an alliance, Italy does not need 1:1 replacement for retired artillery pcs. There is emphasis on modern weapons, the old stuff is sold for peanuts.
The situation is totally different in the subcontinent, where Indian military expansion is forcing PA to raise more armoured regiments. For more armoured regiments, you need more SP howitzers.
For list of PA armoured regiments, see: http://www.pakarmymuseum.com/exhibits/armoured-corps-badges/
Few more armoured regiments have been recently raised.
The most convenient way is to buy these surplus pcs - available in Europe & elsewhere in hundreds. You can equip the IABG's and IMBG's, new Mech. Div. and if some are still left, attach a battery of 6 SP guns with the armoured regiment of each Infantry division. All this instantly adds to your capabilities.
The infrastructure to maintain these already exists at HIT, some spares are being manufactured within Pakistan. There exist rebuild facilities, after 10 years or so, HIT can rebuild these guns again, so they become good 'as new'.
There is the option of upgrades in future, since you have them in sizable quantities, Pak can go for a major upgrade program like Al-Zarrar for the tanks. The 39 cal. gun can be replaced by a 47 cal. gun (as on RUAG M109 KAWEST) or 52 cal. gun (offered by Rheinmetall). The electronics and software can be upgraded as well.
SP guns support the tanks, on the eastern border.