Edit has obvious options for misuse. Mitigating misuse might be complicated but that's no reason not to do it. The feature I've just described is not complicated. The feature might have architectural implications but resolving those is literally the job of software development teams (including design, PM, etc).

I think it's pathetic to say of a feature that it's too hard because there may be some complications or architecture changes. It's especially pathetic to say of a basic feature like edit which exists on other platforms and is obviously possible. This defeatist attitude is, no doubt, partially to blame for Twitter's inability to improve.