Leave the default as "mesa", but this enables parallel Mesa installs.
Sort of inspired by IRC discussion around !7660 (merged). This will need a bit of server support too, but the idea is if you really wanted to keep a DRI1 loader around you could set the vendor name to like mesa_dri1 for DRI1 screens. Other possibilities include parallel runtimes for debugging, certification, hardware-enablement in late stages of LTS OS support...