Also, the geographical element makes BA Gold pretty "sticky" for most UK based frequent flyers.
If you are UK based, its really hard to find a decent alternative to BA schedule if you fly to a mix of European and trans-ocean destinations. I.e. BA make it a lot easier to qualify for status if you are NOT UK nased, as they know that UK based travellers mostly stick with them anyway. Also, another important factor IMHO, a vast majority make BA Gold through corporate travel, i.e. their company has travel arrangements with BA and they do not have any choice in the matter anyway.
In summary, no, I dont think the changes will cause that many people to leave. They may be unhappy about the changes, but they will stick with BA.