It's a brilliant idea and it costs absolutely no dev time.

You know the patch notes they bring out with each patch? That super long list of everything they've added? They could just put a ruler line every page or so.

You want everything up to Line A? That's $30. But everything up to Band C is only another $20. Bargain!.

The added benefit is new players would have to buy all the content from each previous patch before they could buy content from the latest patch.

SE needs this money to keep creating new content, guys. Remember the patches we got up to 2.4? Remember how there was almost nothing in them? That's all a subscription will buy you: 24 man raids, new primals, extreme primals, new dungeons, hard mode dungeons, new classes, new jobs, new main story line quests, new side quests, treasure hunts, notorious monsters, new crafting recipes, new minions and mounts, veteran rewards, beastmen strongholds and dailies.

SE need cash shop money to bring you important stuff like Sleipnir and wedding mounts. There's no way they could create these assets from sub money alone.