Why isn't drag n' drop working like it used to?

I have spent hours on the phone with Apple Support and we have determined that this issue started with OS X Sierra. The only Apple app that appears to consistently allow drag n' drop is TextEdit (contrary to some other comments on related threads). This is a real deal-breaker for me, and I'm getting ready to return my brand new MacBook Pro because I can't "retrograde" to El Capitan and reclaim this capability. I'll just have to take the performance hit and downgrade to a 2016 or 2017 MBP, in spite of the keyboard issues.

Apple is shooting itself in the foot, again. This is a four-thousand dollar machine, and it's worthless without consistent drag n' drop, to me. I need it EVERY SINGLE DAY, MULTIPLE TIMES A DAY.

If I wanted PC functionality, I'd have bought a PC!

Apple Support suggested a workaround involving the Services contextual menu—>Open in TextEdit—>drag n' drop from there. That's kind of like translating text from German to Spanish before you translate it to English! A LOT of the meaning (in this case, ease of use) is lost in the translation!

I iterate: IF I WANTED PC FUNCTIONALITY, I'D HAVE BOUGHT A PC!!!