No question that some of the planes have "outdated" interiors but I am usually flying in one cabin higher of class than I paid for via use of GPU for international flights. So I have to compare outdated UA Business vs modern brand X in economy. Its harder comparing upgrade to GF with a good alternate airline in Business. I pretty much go for the sure thing with UA if GPU is confirm-able at booking.
Use of GPUs on LH used to be a viable option but with restrictions on fare class now, I hardly bother and only for Biz to F as I would take UA E+ over LH Y any day if I had a chance that the upgrade wouldn't clear on day of departure.

But if SMI/J gets his way and ends GF, I won't be buying any Business Class tickets from him if I can upgrade from economy using a GPU. Any paid Biz would be on another airline.