Though you are correct in that what was found was "soft" after being de-minerialized, you are forgetting that the bones in question, from the Hell Creek formation, were reported to have "stunk".
"This shifting perspective clicked with Schweitzer's intuitions that dinosaur remains were more than chunks of stone. Once, when she was working with a T. rex skeleton harvested from Hell Creek, she noticed that the fossil exuded a distinctly organic odor. "It smelled just like one of the cadavers we had in the lab who had been treated with chemotherapy before he died," she says. Given the conventional wisdom that such fossils were made up entirely of minerals, Schweitzer was anxious when mentioning this to Horner. "But he said, 'Oh, yeah, all Hell Creek bones smell,'" she says. To most old-line paleontologists, the smell of death didn't even register. To Schweitzer, it meant that traces of life might still cling to those bones."
The prevailing wisdom now is --- we don't know enough about how such soft tissue structures could be preserved ---not, like you say, "it's only been de-mineralized!" suggesting it had been fully fossilized like most other dinosaur bones, thus "nothing to see here folks" "just move along"...