While most of that is true, there are certain cases where editing a proposal would be nice. For example, my field is writing, and I've had one occasion where I changed the wording in the proposal but forgot to remove a word before submitting. As a writer, it doesn't look that good when you get some writing wrong.
And yeah, it's a silly and ironic mistake to make when applying for a writing job as a writer, but it happens, and I'm sure it would happen again.
Even though it's true that I could withdraw the proposal and resubmit it, as far as I understand how Upwork works, the client gets notified of the withdrawal and the reason for it, so you're either going to look hesitant, or just not very proffesional.
All that being said, even though an edit function would be arguably usefull and make life easier, I don't think it's a must. I doubt it happens often enough, and as you said, it wouldn's matter most of the time, only in cases where it's a writing job and how the person writes is somewhat part of the job application.
Он нащупал в кармане пиджака пистолет. До сих пор Дэвиду Беккеру необыкновенно везло, и не следует и дальше искушать судьбу. Пиджак защитного цвета от него отделяли теперь уже только десять человек.