when I am supposed to fix tech debt? if every week there is another functionality going out that needs to be done yesterday? Do you think that I have to do it in my free time? Why should I even bother existing
Try to lie about how long will it take. Just today I finished* 1-month almost total rewrite of firmware for one of devices at my work. It started as a 1-week small rewrite of part of communication module for a small bug and was scheduled as that. But I've got chill PM and coworkers who will appreciate that now we can actually fix some 8yr old bugs in legacy parts of that code.
* well, now some testing of edge cases and another round of fixes but at least remote code updating works so we can ship those devices...
Edit: "Lie" is call to action here, there were some misunderstanding in comments. Previous start of my comment was "Lie. ..."
Let me tell you the tale of a man named Sisyphus...
I have never in my professional career seen a solo developer rewrite something from scratch successfully in isolation. It almost always turns into the developer leaving, either because they became burnt out being the sole SME, or it fluffed their resume enough for them to find a new job. Everyone else on the team is left holding the bag.
I'm actually more surprised your manager/PM let you do this. Most of the time this happens as a skunkworks thing.
Rewrites should always be incremental, never big bang, and never in isolation.