“Currently there are over 20 uses of the SumTree in Zed.”
Zed’s rope isn’t really a rope. It’s a thread-safe, copy-on-write B+ tree that also holds the project file list, git blame, diagnostics, chat messages and every layer of the display map. A string makes you move 20,000 lines to insert one word in the middle of a file. Zed picked one data structure first and built the editor on top of it. Most editors pick the editor first and spend years patching the data structure.