You don’t need to increase the main chain block size for scaling. I don’t think anyone is proposing that.

For bip 300 block size remains the same it just allows for scaling on sidechains.

Regarding nodes runners vs miners power dynamic. I am not smart enough to comment. I have read some both arguments saying nodes have full control of the network then I read some counter arguments that miners have more control. Not sure which is correct but I guess it’s nuanced.