They do not map the same. Take Larges as an example:
Shiro - 7/17, 16/46, 30/60
Lavender Beds - 3/33, 6/36, 28/58
Goblet - 5/35, 15/35, 30/60
Mist - 2/32, 5/35, 15/45
My Mist 60 is a medium. If I were to be automatically moved to Shiro or Goblet 60, I'd be placed in a large (which I do not want - I want a medium). It would at least be a medium still in Lavender Beds.
But then take my FC's large (LB 33). In every other district, that plot number is a small.
Then you still have the issue of even moving small to small is not the same. Where someone might have a small with a good view now, the same number in a different district might be staring into a wall.
The current system using wards would be better than yours. Leave it in the individual player's hands instead of giving a FC leader control over what other players get.
Remember that for all you used the term "instanced", the wards would still be nearly the same as existing wards when it comes to consumption of server resources. If one FC member is in the ward, the entire ward has to be loaded and not just that one member's property. If the FC only has 10 miembers, that's at least 19 plots going unused.
The ward system itself is the problem if SE can't keep up with the hardware demands to make it function and work for all players.
Who's complaining they can't get a FC room now? No one. They get an apartment instead.
Disputes within FCs happen all the time.
And see Awha's response.
I easily could have done it myself when my alt was the only active member of her FC for 5 months and ended up leader through the auto-dethrone. I could have kicked everyone out and had a second house all to myself instead of maintaining the FC until people started returning and eventually handing leadership to a friend who wanted to be running a FC.
There are other players who have come to the forums in a similar position "I'm now leader of a FC with a house because everyone else quit playing, is it okay for me to kick everyone so I can keep the house?"
Very few FCs are more than loose social groups that fall apart as members have their interest change and leave the FC or quit the game. The wrong person getting leader through auto-dethrone or other means can destroy it.