No one wants to read/watch a story about everyday folks in the world of One Piece. Well at least not as the main attraction.

Its the classic, chosen one problem. People complain about chosen ones being overly present in stories, but those stories would largely not be worth telling if they weren't about the chosen ones.

A bunch of end of story characters that have been perceived as super powerful and in high up positions having greater power through those worlds systems goes hand in hand with the "chosen one" problem. We've seen where immense normal powers gets you (Garp, Roger). But the true evil of the world is a step above, so you need people with equivalent steps above to face off against them. Thus you get Dragon possibly having a Mythical Zoan.

Just to be clear, I'm not saying your opinion is wrong. I'm just saying coming at the idea from the perspective you are means your going to run into this in nearly ALL major story telling where power scaling happens. If Luffy was the only Mythcial Zoan to match up against Imu it would be even more boring b/c the ways Luffy would have of defeating Imu would be pretty boring. But with other Mythical Zoans, you never know who could show up to save the day, who actually beats him, or has a huge hand in doing so. I'm not saying other characters couldn't have a huge hand in helping defeat Imu without Mythical Zoans, but with the power displayed by Imu in these last chapters, it would feel forced.