The bottom line up front is that if keeping your teeth is important to you then you will take the necessary action to keep them. Not easy or cheap.
The stages that you go through to lose your teeth is simple. Just stop working hard to keep them. If you never or rarely floss then your gums go bad while allowing bacteria and viruses free access to your body and over time your teeth fall out.
It’s good that you are frightened about losing your teeth. You will likely work hard with your dental hygiene and have regular dental exams and cleaning.
I’m 69 and have all my teeth. Well a lot of them have crowns and I have one implant. I will never have dentures (NEVER!).
Good luck and good health!