This is the reason a #Shimano XT #derailleur costs more than a Shimano Alivio. On the left you have the whole-body slop of an Alivio M3100 derailleur. On the right you have the same of an XT M8000. There’s noticeable slack in all the pivot joints of the M3100. There’s no noticeable slack in the XT. It feels like a single piece. The result of this difference is misshifts like going one gear higher then back to the desired one, or otherwise some shifts taking longer between gears.
There’s a second video in the original post showing the XT derailleur.
The M8000 derailleur/shifter on my bike are from 2017. The cassette is M7000 (SLX) which is also that old. I’ve done at least 25000km on this drivetrain. Probably closer to 30-40K. I don’t clean it regularly. Maybe once every 2 years. Last year I shamefully fell behind on maintenance and rode it dry almost all summer. This drivetrain still shifts perfectly and new chains don’t skip on the cassette. Point being that if this anecdata is any measure, even old, used compontents from these series might work great, if you find some cheap deal.