I get spam from legitimate companies who don't honor the unsubscribe links. The problem is that many of them use third-party services to handle the unsubscription server so you're feeding the data broker ecosystem with confirmation that you're an active address.
This is an irrational reason. When you click “mark as spam” the sender can configure the email so your email provider notifies them that you marked it as spam. (See email feedback loop).
Further, pixels can be embedded in the email so they can see when you open the email and how many times.
The sender has every incentive to properly handle unsubscribe to avoid spam traps. If you get big enough, users WILL come sign up for your service with a known spam trap email. If your operations are as sloppy as you’re suggesting, your email sending capabilities go poof.
