I’ve dug into the history here and I believe we decided not to use an icon simply because we couldn’t find/come up with an icon to represent components.
This is a screenshot from when the concept of themes and components was introduced (2018). There’s no icon for the components tab for the same reason.
Given all the recent changes we’ve made around themes/components, I’m not sure if it even makes sense to have the convert button anymore — I don’t think there’s a use case for it these days? I certainly don’t remember the last time I needed to use it.