It’s still much better than the other models.
For a start the price had not changed since launch except for some regions ( grumble).
It’s not only about server costs it’s also about profit - it always has been, let’s not be dimwitted naive.
Actually - it’s more popular among publishers now, just given a different name.
First we had the free to play era which has almost ended, where the base game was free but there were several ‘inconveniences’ built in to point you toward an inflated cash shop.
The other model was in episodic model where you bought the base game and then essentially content patchs at a fair whack.
Games are now moving towards the premium membership. games like SWTOR remove all the inconveniences and gave expansions ‘free’ as long as you sub.
In other parts of the industry it’s known as ‘software as a service’. For instance instead of buying MS Office every release (if you choose) you can essentially sub and get included upgrades. Stop your sub and you have nothing.
Early MMO games were pioneers of this model; there’s only so much whaling you can do until all the whales are dead, ehm penniless.
As for £600, well that’s over several years. It costs me £9.99 per month, or less if I sub for longer. Looking at that as a daily rate it’s £0.34 per day; it’s still not much, even if I only play it a few times a week. How much does a pub visit cost and there’s no hang over! Renting a movie for a few hours cost several pounds.
I do however think blizzard are taking the ‘mickey’ over their expansion prices.