> What is proposed in glep-55 seems to aim to solve both issues at the 
> same time (it isn't stated) by switching file extension every time the 
> eapi is changed. This is slightly against the principle of the least 
> surprise and apparently is disliked by enough people to lead the 
> situation to be discussed in the council.
> 

Instead of switching file extension every time the eapi is changed you
could also increment it only when a new EAPI breaks sourcing the ebuild
compared to the requirements of the prior EAPI.
(This way you'd in fact split EAPI into a major- and a minor-version.)