The problem is that QT Player 7 is 32-bit app.

However, Apple said that High Sierra will be the last OS that supports 32bit apps, which means it still runs 32bit apps
And you are right, the "update" is one of so many software "updates" from Apple that are actually "downgrades" because they removed features (that users often relied on with their workflow). I also still using QT Player 7 and I feel your pain.
Hope that helps
EdgarRothermich - LogicProGEM.com
(Author of the "Graphically Enhanced Manuals")
http://DingDingMusic.com/Manuals/
'I may receive some form of compensation, financial or otherwise, from my recommendation or link.'