Why? There are tons of benefits. 4k offers better quality for obvious reasons and down sampling it to 1080p makes the 1080p quality better.
S log provides more head room for editing your footages. You can get more dynamic range from it.
Huge benefits. That even I make use of. You think I buy stuff with stickers and don't use its features. I do. You have no clue.
D750 I assume doesn't have those features which to me and many video users is a downside.
Mirrorless offers far better video than dslr because of things like that and the quality and tech they have.
Ibis is another thing that's great. And makes a difference.
Don't think you have even tried it?
Maybe you should just record video on your iPhone?
Translation for @twist if you're not prepared to pay through the nose for the latest equipment like Jonney, regardless of the actual quality of your output, then you might as well just use a mobile phone.

