You're talking about optimization. In your opinion emails with fancy layout and "on-brand" design will get a lower level of engagement than plain text with well-written copy. You may well be right (in a lot of cases.)
But you don't know that.
It's a guess based on your biases and assumptions. This is why it's critically important to measure things when you optimize them. Believing that you should engage with your customers in a way that you like because "they're just like me, so they'll like what I like" is an easy way to kill your messaging dead. Optimization of something like email is a matter of constantly measuring, refining, and measuring again.