Even if an app doesn't need more than 4GB of RAM it can still be developed as a 64-bit app. 64-bit apps can use more than 4GB of RAM, but that doesn't mean that they have to use more than 4GB of RAM. The only way that a 32-bit app is using less RAM than a 64-bit app is if the two versions are designed differently, which is part of the reason that 32-bit should die.
If we move solely to 64-bit then developers don't have to maintain two different versions of the same program, or code the program around the least common denominator (32-bit) if they don't want to maintain two separate executables for the same app. It's the same reason developers should drop last generation consoles once enough people have migrated to the new generation consoles.
At least with that console example developers can say, "There's too much money left on the table by not supporting last generation consoles." If we move entirely to 64-bit apps and operating systems then we don't have to worry about that anymore, and developers can maintain a single development rail which is better for everyone. And again, just because it's 64-bit doesn't mean it has to chew up RAM.
P.S. Moving to 64-bit only operating systems and apps means that there will be less resources taken up by the operating system as well. This is because an operating system that only supports 64-bit apps won't have DLL files and resources for 32-bit apps to use. So it's also a storage saver, although the storage savings is likely not much. But still, it's another reason to move away from 32-bit apps and operating systems.
X86 will never be 64bits only… 64bits is an extension over 32bits like SSE for example.
This alone will guarantee that X86 32bits will never go away unless you move to another type of CPU architecture that is exclusive 64bits.
I can understand your others points like less maintenance/optimizations to be done in code or even less storage used with 32bits libraries but OS won’t stop to ship with 32bits because at the core x86 is 32bits and that will never change.
I move to 64bits apps when there is some reason to do it… like MS Office that allows more rows in Excel (probably due the memory limitation in 32bits) but it doesn’t add anything in terms of performance over 32bits… and the apps I tested in 32bits vs 64bits showed the memory consumption increase a lot in 64bits app.
BTW in Virtual Machines I use only OSs in 32bits due the lower memory footprint… it uses less memory so I can setup my VM using less RAM from the host.
IMO I don’t see real advantage in that choose by PCSX2 to go 64bits only… in fact I believe a lot of old machines that can run PS2 games via emulation will be left behind… that include some lower specs machines only used for emulation.
I read a lot of dev post and I could not find a reasonable reason for the change at all (in fact they don’t expect any advantage in performance and some even a lose in performance at beguiling and in long term to catch the 32bits version).
But hey it doesn’t affect me so I’m fine… I just find weird because the 32bits is not a issue at all.
When you will choose a new App ask yourself? Why you need 64bits? If you have a reason or don’t know then choose 32bits… you will be better served.
PS. The console compression is not really accurate… we have a new hardware each generation in console… PC doesn’t have generation… you don’t have a point where everybody has the same exactly machine hardware wise (fixed config).
PS2. The decision seems more political than technical to be fair reading stuffs.