Downmixing is a pretty straightforward affair. You have 6 channels, you need to go to 2, so you just average 4 signals per channel using some weights.

Good media players (Kodi) allow you to change those weights, especially for the center channel, and to reduce dynamic range (with a compressor). Problem solved, the movie will be understandable even on shitty built-in TV speakers if you want to do that for some insane reason.

The problem is that there are "default" weights for 2.0 downmixing that were made in the 90s for professional audio monitoring headphones, and these are the weights used by shitty software from shitty movie distributors or TV sets that don't care to find out why default downmixing is done the way it is. Netflix could detect that you're using shitty speakers and automatically reduce dynamic range and boost dialogue for you, they just DGAF. But none of that is the movie's problem.