I see where you're coming from, but this tool is still valuable in a number of cases that I don't think you're considering.

I recently got a new mechanical keyboard, which had special "layers" for punctuation. "Common" punctuation like periods and commas are still there, but other punctuation like curly braces, brackets, etc. require new key combos. [1]

I agree with you that a lot of the time programming requires modifying rather than composing, but not all the time. In particular, I blog in Markdown, which requires a certain amount of extra symbols that I wouldn't normally be typing. (Goes hand in hand with the previous point).

[1] You might be wondering, "Why get a new keyboard, if you're typing slower with it?" The new one is easier on my hands, wrists, and fingers, as well as being an actual pleasure to use.