Morgoth : I really do not think a merge would effect the economy, if anything it would just bring the small minority of UK server to US that refuse/d to move, that makes it bit more people and as we all noticed lot of people = cheaper stuff.
Even if UK players who moved to US use their UK accounts again on US I really don't see why that would effect the economy. (Keep in mind if even a merge happens it will be in a far far future where most of US will be 93lv+)
The rest points you guys raised I can agree on but still it is only temporal in a year let's say OP shells will be all around again.
Either way considering majority moved over most of the players did not really care about their UK accounts, and then leaves the minority that just want to move over.
Maybe they could arrange account transfers and also charge something like 20 euro per account transfer(This is an example don't shoot me please) that would make the minority happy and the company happy for making money and would also save a huge database merge that would take ages.