Players connot join the party while you are engaged in combat

[Kurokikaze]

This is how D&D Online handles it. In fact, if the entire party logs off and comes back, the party' actually remains intact even several days later, until the current party leader breaks it ( leader reverting to the first person to log on ).

It's actually how the vast majority of MMOs today handle D/Cs. Unfortunately, FFXIV is a hipster and cannot be asked to incorporate such mainstream technical features.

To be honest I don't its the devs themselves that don't want to do it. It's just that the spaghetti code that is FFXIV is so overly complex that the most mundane fix is made out to be monumental.