It is fairly straightforward to extend gawk since they added the gawkextlib functionality to regular gawk (vs xgawk).
Here's the readfile extension, for example: https://github.com/gvlx/gawk/blob/master/extension/readfile....
But, you're right...there aren't many extensions, and little activity around adding new ones.
The @include and @load directives are extremely useful for shipping your own customizations, but I prefer the maintenance priorities of the JQ maintainers [1], who understand that powerful builtins are what burn into user's minds, making a tool mentally indispensable.
1. https://github.com/stedolan/jq