Cell Breathing doesnt affect capacity as much as it affects coverage in my experience. If you design your link budgets and cell sizes based on worst case Ec/No you are prepared to accept in the network (and due to the pole capacity equations you basically run out of power and cant push any more calls on even if you increase available power once you pass about 70-75% load) you can still realise the capacity you expect (say around 60-70 ccts/cell/carrier) at full useable load of 70%, over the full footprint of your desired cell coverage.
I have seen this live in networks previously.
So the capacity gain of UMTS over other cellular systems still holds. Sure, load as impacted on it a little, but 50% load does not result in 50% capacity reduction, rather it results in a coverage reduction – which if your inter-site distances and inter-cell border designs are good you can tolerage without harm.
Prediction systems like Forsk’s Atoll can run simulations that show you what happens in these circumstances so that you can design around the impact of them.