DX12 does have minimum hardware requirements though as Windows 10 actually has a minimum mobo needed to run the OS (which also comes with its own overhead over Win7).
In my tests of Doom on Vulkan (running Win7 on an i5 Ivy Bridge) I actually received no benefits what so ever, the game isnt CPU bound its GPU bound as are lots of games so you’ll still quickly reach the limits of your GPU if its as old as those CPUs.
I havent been able to find much info on Vulkan in Unity, most of the stuff Ive seen is marketing hype and doesnt go into specifics like which Shader Model level it supports, if it goes all the way up to SM5 as UE4 does support Vulkan up to SM4. Its quite possible Unitys implementation is limited too, Im just not sure, it does look like they support Vulkan on PC for SM5 but it would be nice if they didnt hide pertinent information behind hype especially in regards to GDC (not really a press event).
Ive actually seen very few games fully utilize DX11 to its full potential because of the focus on consoles and mobile, thats not to say the OP is wrong just that DX11 is actually 6 years younger than Sandy Bridge.
I would prefer to see Vulkan support SM5+ (in UE4) over DX12 myself because of Microsofts tightening grip on their ecosystem, it actually has nothing to do with Unity at all tbh!