1) Do you acknowledge that some characters are going to roll well on hit points and end up with above-average hp? If so, then you must acknowledge that some GMs are already dealing with PCs that have higher hp than normal, within the default parameters of the game, and any concerns about PCs with high hp have been dealt with by many GMs in the 30-plus-year history of the game.
2) Do you acknowledge that retraining hit points takes 3 days for 1 hp? If so, you must acknowledge that retraining hp is a very slow process (much like gaining levels and hit points from leveling) and the GM isn't going to be "surprised" that in one session the PCs have low or average hp points and in the next they have above-average hp, and therefore the GM has time to adjust to how much tougher the PCs are and compensate for that in a slow, dynamic way.
3) Do you acknowledge that the GM is in control of how much downtime the PCs get? If so, you must acknowledge that the PC's can't just decide to stay in town and max out their hp; the GM is going to push campaign events that prevent the PCs from having three-month training montages to gain +30 hp each.
Conclusion: retraining hit points only allows PCs to reach a level that some characters already reach randomly, that it is a slow incremental change that can't be sprung on the GM, and the GM is in control of the process, and therefore taking a "sky is falling" attitude about how this could wreck a campaign is an inappropriate overreaction.
I'm sorry that you didn't like my humorous take on the situation. I guess I'll add "employees can never make jokes and must be serious at all times" to the secret Paizo bylaws.
GM: You have three days of downtime available.
Barbarian 13: I retrain hit points and increase my total hp by 1! Booyah!
Wizard 13: I create a scroll of limited wish.
Barbarian 13: *cries*
Poor,poor barbarian. The whole caster community hate you and your hp retraining stuff. However, I think that such rules are not destabilizing, any dm can handle it with minimal effort and can only do much good to the game. And at the poor barbarians, indeed.