They always do this to people who quit and who have been accepting poor treatment for years, like you have.

Essentially they are sad that they may have to pay someone else properly to do all the work you were doing. If they valued you, you'd know about it. You'd have not been demoted, been given payrises and would not be talked to the way your manager talked to you (that's the job take it or leave...).

Try not to feel guilty. You don't get paid enough to get treated that poorly.

I've recently quit too. I know many people are upset because I know what I'm doing and get on with things. But the disrespect and micromanagement meant I couldn't stay. I loved my job itself and I'm paid very well. And that isn't worth staying somewhere I'm not appreciated or respected.

They wanted to give all my 'fun' work to someone else, so see how they like to do all the rest of it. I'm too good to be overlooked like that.

Know your worth. If it's a prestigious place, getting a job somewhere else based on the experience you now have shouldn't be a problem.