Hi Steve
I can see what your saying but my way of thinking is a lot of people will attempt to stick to 15KG and not want to pay the extra at £5 per kilo.
On a 200 seat plane if 10% went to 20KG via the excess method that would give 20*(£5x5)*2 = £1000
If they charged £7.50 each way and 50% take up the offer that would give 100*£7.50*2 = £1500, I don't believe to carry an extra 1/2 tonne would cost £500 more in fuel and I wouldn't be surprised if the take up was even higher. I may be totally wrong but I think it makes business sense to offer the upgrade.
I think we will just pack for a week and use the laundry services in resort to keep our weight down, either that or wear all our cloths at check-in.
Dave