What are some consequences of living 12 years with unfinished root canal?

A few: Pain, swelling, infection, loss of tooth. Can't say what condition your tooth is in or if it can be saved.

Incomplete RCT: These types of situations always result in losing the tooth. Worst case scenario is an infection which can destroy bone, other teeth and spread to other parts of the body, causing serious complications. See if the rct can still be completed and the tooth restored, or have the tooth removed. A walking time bomb!

Significant: You may be harboring bacterial infection in your mouth which can spread throughout the body and affect other areas/organs. You may be disintegrating the bone (resorption) around the tooth to the extent of it not being able to be restored. You may be allowing the tooth itself to become decayed to the level of non-restorability. None are good. Get it checked and move forward. Good Luck.

Tooth loss: If root canal treatment is not done, the tooth may have to be extracted.