It's ridiculous lol.
Midjourney is optimized for beautiful images, while Nano Banana is optimized for better prompt adherence and (more importantly) image editing. It should be obvious for anyone who spent 20 minutes trying out these models.
If your goal is to replace human designers with cheaper options[0], Nano Banana / ChatGPT is indefinitely more useful than Midjourney. I'd argue Midjourney is completely useless except for social media clout or making concept art for experienced designers.
[0]: A hideous goal, I know. But we shouldn't sugarcoat it: this is what underpin the whole AI scheme now.
It is what has underpinned all of human progress towards automation. It isn't a bad thing. Every time we automate something the luddites cry out about the coming mass unemployment. It has never happened.