Permanent UU building

[Pouakai]

I had an idea when playing ciV today that you can build your UU, after it becomes available, for the rest of the game, even after it becomes obsolete. Granted this becomes useless in late game, but I find myself not wanting to research certain techs because I can't continue to build my UU. Also, maybe allow the ruin promotion of Scout > Archer to be done conventionally.