64 bit also opens up the possibility of more race/class combos. I seriously doubt they'll add anymore classes at this point, but I think new races and/or combos are definitely on the table.
There's no reason you couldn't add another class, race, or deity in a 32-bit application. EQ could have done this years ago. There's probably just simply too much code you'd have to change in a reasonable amount of time, and doing so is error prone when you don't have a team of engineers working with quality assurance, artists, designers, etc - plus bean counters would have to see a reasonable return on investment for having developers spend time that way.
Technical explanation below:
You can still use int64 in a 32-bit program, it is the same as doing char bitmask[8] and making helper functions to compare classes or races.
EQ has had int64 health in a 32-bit application for many years now, because the keyword doesn't care what arch it is running under.
64-bit only means you have a 8-byte pointer type, and can address allocations of memory up to 0xFFFFFFFFFFFFFFFF (8 bytes), instead of 0xFFFFFFFF (4 bytes), as well as the instruction set supporting better ways of handling floating point math, intrinsics that natively parallelize instructions through the instruction set, and other optimizations that EQ will likely never utilize.
The only reason to do this is the reason outlined in JChan's post; they are simply running out of addressable memory for the application, even with a 4GB ceiling - and this is commonplace for most 00's-10's games that want to push graphic fidelity and utilize modern GPUs, but cannot because the textures and other addressable elements required to store data are simply too small.
The other issue which will likely follow is an upgrade of DX9 -> DX11/12, but that would require a complete rewrite of their graphical engine because of their heavy usage of the fixed function pipeline versus shaders that drive the game's integration. It's why DX9 was a simple upgrade for EQ, but DX10/11/12 is 'out of reach' for most teams that have downsized their staff but still use in-house engines. So seeing Daybreak do this, unless it's already done for them, is likely out of the question and this is the next best option.
The reason for the above comment is that EQ has historically 'gotten away' with tricks that allow them to bypass the need for that graphics engine / directx api rewrite. DX7/8/9 are 'pretty much' interchangeable... if all you use is the fixed function graphical pipeline. However, you're missing out on modern optimizations like texture pooling, caching, and other GPU-offloaded tasks such as render call batching. EQ doesn't suffer as a result of not using those.
However, they hit a roadblock with graphics fidelity and memory - if there's too many different assets loaded 'on-demand' and they run out of addressable memory (GPU or otherwise, it doesn't matter if the application cannot tell the GPU where it resides in memory), they cannot work around this without converting the entire application to use a 64-bit address space.
This is, of course, a workaround - it will still require users to have more memory to run the game. Thankfully most have 8GB of ram or more in 2021, as Windows 10 simply requires that to run these days. And whatever you can't put in physical memory, putting the game's assets into the paging file ('swap file', if you would) becomes a viable option, and the OS will natively handle putting the assets it can't store in physical memory into swap memory as the game can address that memory, even if it's on the disk drive temporarily.
I think it's a brilliant solution, personally, and it's the one I would pick if I was in their situation. In fact, I am actually in a situation like that right now in my professional day to day, and I'm doing it alone - at least Daybreak has a team doing it with them, I'm doing it alone!
EQ doesn't require a demanding GPU currently, it's a bit heavy on draw calls at most. So they can use the DX9 solution for now, and maybe stuff a lead or two in a room together, or hire external talent to write that graphics upgrade, and in the meantime, the game won't crash due to out of memory - it may just be 'laggy' for those with 6GB or less RAM on Windows 10.