Cannot get rid of forced featured image

So many otherwise interesting themes are unusable because by default they produce a forced featured image that cannot be turned on or off or adjusted in the customizer. Twenty-Fifteen, like so many other “nice” themes, forces an arbitrarily blown up & cropped version of my featured image into the top of each post and page as a header on the excerpts and on the post or page. This image header conflicts with my selection of images on my post or page, is redundant, not capable of being controlled or disabled in the customizer. The same problem is found in lots of themes up to the current year 2021. Very very unhappy, and upset that I can’t upgrade my theme without totally wrecking my already-prepared excerpt and image layout.

Ergo, Automattic, PLEASE ADD CONTROLS to the CUSTOMIZER to allow us to control these forced featured images that come with so many themes.

Meanwhile, if anybody has an easy way to fix this, like a plugin, please let me know. Thank you.