The complaints you cite about Blue Ray disks and players were things I read about in the early days when Blue Ray first came out. I find the exact opposite is true now. Besides, depending on the movie and disk, Blue Ray and 4K Blue Ray disks offer several audio formats to choose from and the audio is uncompressed. I would really like to see your source claiming "thin sound".
Streaming media cannot match the sound quality of a 4K blue ray disk. If your reference for your information is Youtube and Forums, then it's just opinions. That's fine. I guess you can say it's real world experiences. Everyone has an opinion I am sure. If that is the case, then my opinion is my OPPO UHD player with a blue ray blows away anything I stream from Apple, AppleTV, Roku, Netflix, Hulu, and Amazon.
Just one quick search revealed several articles. Here is one that lightly covers most topics:
https://www.whathifi.com/features/4k-streaming-vs-4k-blu-ray-vs-blu-ray-which-best
Good points. Poor What HiFi review comparing a poor quality 'Amazon' stream. Maybe if they'd compared a iTunes 4K Dolby Vision film, things may have been different.
Yes your Oppo is superb - as a disc player, it's the best BUT the problem there is that you've got the best player, so it's not a true comparison to the rest of the disc player rubbish out there. Remember an Apple TV 4K is £179, your player is £650 - 4 times the price. In a way, it's like owning a top spec, brand new Ferrari and claiming that every car on the road is absolutely superb - because that's the only car that you've driven
But hey, each two their own. iTunes downloads suit my lifestyle - travelling - watching film & video on my iPhone X, sharing with friends, watching on my MacBook Retina and once a week on my Apple TV 4k. in the digital age, it's all about multi platform. (And no, here in the UK, UV certainly does not count).
Firstly, I won't have discs lying around my beautiful home - I had that growing up in the 1990s. And here in the UK - none of my favourite new films are on 4K disc. Most aren't even making it to BluRay. An AMAZING new film from SONY called 'Paris Can Wait' released this January did not even make it to DVD release here in the UK let alone BD or 4K BD.
So in essence, it's very very very well saying that disc is wonderful BUT it's not of much use is the film or surf documentary is not released on a physical format and my gosh, that is exactly what I have found in 95% of my cases!