Why the claiming system will be removed
We decided to remove the claiming system from FFXIV because we decided that claiming would become a troublesome spec in the future development of the game. When considering large scale PvE content involving multiple parties and PvP between large numbers of players, the spec that claiming occurs on a 1:1 basis becomes a nuisance. Also, instances will become more prevalent in future content. The concept of having to claim opponents to prevent being disturbed by other players becomes less and less relevant, so we decided that claiming is unnecessary.
Implementing future content that requires multiple groups has been done to death in plenty of other games while keeping a claim system that doesn't allow every person in the area the ability to freely attack a mob. Its only an issue of you have no faith in your dev team or your dev team is crap.

The claim system did not need a drastic adjustment. You could have implemented a Group Alliance, Warband, Teamup system where x amount of groups could be combined together into a a raiding party without a drastic claim change, all groups in the larger raiding party would be able to attack the same mobs.