It varies by district but my wife is a teacher (now) and she has worked in several schools where the special education budget met or exceeded the non-special, while serving only 3-5% of the school's enrolled children.
My wife also teaches these sorts of children and the vast majority of them seldom improve beyond a 5th or 6th grade level, and many of the "smarter" ones with milder disabilities end up in jail at some point after being graduated (most typically for sexual, theft, violence, and drug offenses) out of the system since they can no longer be failed, yet they basically can't truly be taught how to properly function in society since they can't even properly comprehend laws or how they work, as they only have basic reading capability and comprehension of even simple children's books.
Closing institutions insteading of reforming and modernizing them for these sorts of people was indeed a mistake.
Leave them behind and underfunded like in the past? Lock them up?