I'm not saying it can't be done, I'm saying it would take a lot more effort (by a huge amount, plus more in prototyping phase of development when you're trying stuff out), and in the process you will likely lose many of the advantages of native UI that were mentioned (consistent behavior etc.).

Take for example that "Jump to" text box - if your native UI toolkit doesn't let you add icons into the placeholder text like that (does it even have placeholder text natively?), you'll have to deal with drawing offsets, caret position, or possibly nested components, overriding focus outline, in the worst case resizing the stuff when focused, and other stuff that makes the whole endeavor not worth it due to the added complexity (assuming that text box works how I think it works, I haven't used Slack in a while).