My main objection to using tabs has always been that the Tab key is heavily overloaded: it navigates (one Tab press to move the cursor to the correct indentation point), selects (tab completion), and it puts a variable-width character into my monospace-defined text file.

I'd rather drop the insertion part and have my editor handle adding an appropriate number of spaces.

Let me add this: there's actually nothing stopping you from displaying a four-space indented file as a two-space indented file. Just parse, replace indentation-dictated groups of four spaces with two spaces, you're done.

I don't know of a plugin that does this for $your-favorite-editor but it's, y'know, software. There's nothing which prevents it.