I think you make some good points but I think you're off the mark in some places.
Not all screen time is equal. I agree letting kids watch continuous streams of content is bad, it can and does cause an upsurge in mental health issues, it does provide a source of dopamine that can become easily relied upon but...
Screens can also be regulating for SEN kids.
You ask how they regulated before screens. Self regulation isn't a skill you develop simply due to the absence of screens. Before screens were more commonplace a lot of SEN needs just went unnoticed or uncatered for. A lot of needs were unmet. Children were told off, ignored, or labelled naughty because that dysregulated energy was directed elsewhere like self injurious behaviours, harmful behaviours, destructive behaviours or internalised. It's not a case of these children just learned to pick up a book and read to calm down instead or went to pick up a toy.
Screen time does give dopamine, and it's important to understand the role that dopamine plays in emotional regulation. It can take children out of crisis states. It's a very useful tool. It can also be a welcome distraction when the world around you is chaotic, it can be a really helpful communication tool, not just in AAC but providing gestalts for our gestalt language processors who otherwise would have been written off as non-verbal for life. It can help caregivers understand needs from our children with idiosyncratic speech patterns.
In short, in moderation, it's not the devils work, and it should be moderated, but with children which it is an essential means of communication and distraction a bit of grace wouldn't go amiss.
It's easy to judge SEN kids, because the world holds them to neurotypical standards but ultimately that's unfair and unrealistic.