84-85% Commonality was a driver, but also the original purpose of the upgrades was to take old airframes and save money from the refurb process. Obviously they went the new build route and costs increased but it is probably better after the beating the W has been taking over the last 14 years. I think everyone is a fan of the H-60s (including the Marines) and the UH-1Y is a good platform that is still working some kinks out.
There are people out there that say we should've gone to an all 60 fleet. I think the 60 could do the Yankee's job, but not the Zs. I will say however that the Y does look like it has greater fields of fire for it's door guns, with the crew chiefs shooting out of the cabin vice a window just forward of the crew compartment. Once you strap on the overhanging wing stubs to a 60 it starts to get in the way. Unless mods are made to put the gunners out of the main cabin door, then it becomes a cluster trying to insert a larger amount of Marines with a door gunner in the way. The Huey typically doesn't carry a lot of pax, mainly SOF, snipers, or an R&S team prior to inserting a larger force, so it's not as difficult. The wing stores completely obscures a door gunners view, and many a Cobra pilot have been saved been a quick swivel of a crew chief on to some clowns trying to take pot shots at a skid section. Fixed 20mm on a 60 isn't an answer to a larger caliber weapon system for an attack or utility helo, off axis guns are a huge advantage for pull offs and for mutual support. I will say that I would prefer a 30mm round with a better fire control system, and due to the cockpit set up on a 60 I would find that pretty difficult to employ effectively, or at least require good CRM with the entire crew.
Multi-mission set ups are a good idea, but in reality I would think that just due to the power requirements for carrying a larger number of pax, full bag of gas, ordnance (a useable load of HF, 7.62, .50 cal, HE rockets, expendables), a good fire control system/sensor, it would start to have power issues, or at least require really good reverse planning from the objective to identify limitations prior to inserting or engaging targets. Another thing to consider is do you really want a 60 putting it's aircraft in harms way to engage targets with a large number of pax in the back? If that's the case...do we take the pax out? Then why do we have a utility aircraft trying to do an attack aircraft's job? Self escort is a great idea in theory, but not at the risk of endangering more Marines in the process, unless it can be mitigated appropriately. Lots of rabbit holes to go down on this topic, and it's commonly talked about in the Marine helo community...and yes Bell does not have a great reputation among Marines right now with how some of the upgrades stuff has been handled.