Hi,

b) to include new drivers (for old hardware) if they arrive (probably happens rarely but there can be).
   I know of at least one driver which won't be able to support DRM....

Hmm?  I seriously doubt that.  There is always the option to use a
shadow framebuffer, then convert from standard drm formats to whatever
esoteric pixel format your hardware expects.

Been there, done that.  Have a look at the cirrus driver.  The physical
hardware was designed in the early 90-ies, almost 30 years ago.  These
days it exists in virtual form only (qemu emulates it).  Thanks to the
drm driver it runs wayland just fine even though it has a bunch of
constrains dictated by the hardware design.

take care,
  Gerd