I actually have no problem with taxing retirement income.
What I have a problem with is changing the rules after you've entered the game. This would not affect me a lot. However, there are a lot of people who plan their retirement income based on tax liability in the state in which they plan to retire.
If Illinois (please, don't...) wants to tax retirement income, let them do it going forward. However, it should be in the context of understanding that many of its citizens have made plans, difficult to alter, based on promises made in the past.