>This is the first and only requirement.

Sorry, but in practice a couple more requirements:

You have to find enough volunteers who are willing to write to your pet language requirements that PG / DP can afford to kiss off the volunteers who are still interested in writing in other formats. Volunteers may be interested in writing in other formats supported by the rest of the industry, for example, because they may not want to be “locked in” to solely writing for PG.

You have to find tool-makers including editor creators to make the tools that make writing books to this standard reasonably pleasant and relatively foolproof.  And these tools need to be supported on all the major OS’s.

You have to find a systems “build master” who is willing to write the tools and actually support them to accurately compile your markup language to the formats which customer’s devices actually support, including html, epub, and mobi, and who is actually willing to fix these tools when they generate broken output.

Perhaps most hard, you have to find someone who is actually willing to write support manuals and online help for all this stuff.

And all this has to be done indefinitely into the future, for at least the next couple decades, so that PG is not left with an incompatible and unsupported pile of rotting compost, wasting 100,000s of volunteer hours.