Omg one of mine is obsessed. They definitely have too much, but they do listen to a lot of music, watch documentaries and do reading eggs (as well as watch crap and play games.)
I find if you brace yourself for the tantrum and grumps it passes quite quickly (well 5-20 mins) and they find something else to do.
A visual timer to count down when it's going off and warnings before hand so it's not sudden is also fab.
My lot know if they want to stay on it then it will go better for them if they're doing a learning app number blocks, reading eggs etc.
I think balance is key. Screen time isn't inherently bad (light screens and being sedentary for long periods is though.)
We have no screens after 5pm on school nights to help with bed time, but it's a free for all before school as I need the extra sleep.
you have to do what you have to do to get through the day and sometimes that involves a screen that will keep them in one place not trashing the house for a bit.
I love the weaning advice about looking at a child's intake over the week rather than day by day. Maybe you'll have one day where they're on for hours but in the same week they've had a whole day of no screens and running about + a week of school and clubs etc. Whilst in my heart I'd rather we moved away from screens more, I think we all sometimes need a chill out zombie day and that's okay too. OH and I are very different in this respect but have reached a happy compromise.