I'd like to be able to remove a text object's style (in other words: unlink the text block from the style), but still keep the text's formatting as is.

What usually happens is that, when you create new text blocks, for some reason, they automatically get the latest text style applied. If you don't pay attention, then you might create many text objects and format them manually (because they don't need to be linked to any specific style), and at some point, you realize that all these texts are linked to a certain style, and that's not what you intended to do. So at this point, you might want to unlink those texts from the style, but doing so, you also remove all manually applied formatting, and you have to do it all over again, which is a PITA...