I've been on both sides of this table multiple times, as the IC and as the Manager of an eager IC. Here's a list of all the reasons why I as your manager would also flat-out say No to this situation. (These are of course heavily tainted by my own recent experience of trying to coach a mid-level dev through a very similar problem)

- "Pretty much working" means all the fun stuff is done and the actual hard thing is left to wrap up. It's a useless estimate that only accounts for your coding work, which is usually the smallest amount of work performed on an integration feature like this.

- It's a rewrite so we've gotta do a full regression test on every piece of data that thing pulls back. Since it's old functionality it's not fully covered by our automated tests, so this goes to QA. Our QA team is overloaded so this unauthorized, not on the roadmap project now needs to jockey for priority with things that Marketing is literally making artifacts for _today_.

- "It's already built" isn't really a justification for a priority change, so now I'm in the awkward position of changing priorities for a non-roadmap task and justifying this to every single stakeholder who is respecting the process, or telling you it'll be 2 months minimum before QA can even think about it. Either way no one is happy and now I have to worry about you going rogue again and trying to work channels around me to get this thing shipped out of band.

- It's a full rewrite and going through manual QA, so it's nearly guaranteed that critical, but undocumented business rule fixes were missed. Somewhere in that library is a weird function holding up the world, but it was "obviously cruft" and left out. There's a good chance we won't find the issue until it has already polluted a ton of Prod data. That's why I won't let you do Developer QA. You've only been here a year and this service predates you, me, and the rest of the team, we literally have no context.

- If the client finds out we did a full rewrite, they too are going to do a full regression test on their end. Do you know the size of the shitstorm this is going to bring on us? Every single question, problem, feature change, bug, enhancement, communication, _everything_ we went through over the last XX years since we built this integration is going to resurface. I get re-litigate every. single. thing. "Since you're working on our integration can we get XX, XX, and XXXX?" (each is a sprints worth of dev time minimum), "YYY isn't working, did you guys break it again?" (it's always been broken but now someone gets to spend 3 hours in Datadog pulling logs to prove this).

- I've been using the "Rewrite This Library" and "Refactor That Service" projects as leverage to negotiate for more budget to bring on 2 more headcount so that we could actually do those rewrites with proper time and space. You talking about getting 80% done over a weekend has completely undermined the work I've put into this effort, and at the same time didn't remove the Refactor issue from my backlog. Now I will essentially have to shit-talk you in my own 1-on-1s in order to regain lost ground. "sfn42 is a decent developer but he just doesn't have a lot of context to what's happening outside his role. Needs more time in the oven before he gets the bump to Sr. Maybe I can pull him into more planning meetings so he can start growing in this area" -- congrats you just got invited to 6 hours of meetings a month regarding work you won't perform.

- In 6 months when our team is planning out some future work that's just way too much for the headcount & timeline we have, and you bring up "we could really use another Sr. Dev or two, any word on our headcount request?", I might reply politely with a "still no word if we can pull that off this quarter", but internally I'm wondering if the pain of bringing a new dev up to speed is less than the pain of working with you.

- Lastly, the most petulant reason, you were told No last week. I'm sorry you lost a weekend to this, but a No is a No and I need you to understand that. Other things are happening at this company outside the scope of your purview.

Again, this is all drawn from my own experience. I had a mid-level dev show me a huge refactor he started on the weekend. He was convinced it was almost done, "just a few small things left" is an exact quote. However I knew that this part was literally the smallest bit of the effort. I was seeing at least 3 months of work across 4 departments before it would actually be Done, in Production, and working to our satisfaction.

If I had the space I would normally be just fine letting the young fella just experience that pain. Make him do the scheduling, put him on point for everything, and just let him spin on it for a month or so. I did not have that time and space, so instead we spent a few hours white boarding out the rest of what needed to happen, and thankfully he mothballed his project of his own volition.