I still suspect that once the new aligned TP years are in place, it will change to 5k for renewal. Not sure if they would increase the threshold for first timers or just keep them both the same (or even 7k for first time, 5k renewal)
3k TP is a bit "too easy" to achieve for some - a couple of not too expensive TP runs, and/or a few US flights (or a few of the favoured SOF 160TP runs) can push you over that for potentially as much as a CW or two, or a F fare.

It may cut back those who can do that final push for GGL, and just leave those with natural J / F flights who can earn 5k without breaking a sweat.
Would then allow BA to concentrate on a smaller, more exclusive pool rather than having sub-sets of GGL where some are favoured above others (although I expect there will be benefits for someone who earns say 10k TP compared to someone earning 5k, allowing for other considerations).