The vaccine is not useless, it's just not a condom either(if you'll forgive the metaphor). It can't prevent you from getting Covid, it's to help you to get mildly sick and prevent you from getting really really sick and dieing from it.
It's effectiveness just doesn't last forever because the human body's lymphatic system and viruses are a complicated thing. It's not a one and done deal. Everyone's body is different and complicated and the effectiveness of a vaccine can wear off, and viruses can mutate. Eventually the booster shot's effectiveness will wear off and there will be another booster shot most likely for anyone with a compromised immune system or if a new virus strain of Covid becomes more deadly.
The vaccine does not corrupt your immunity. The Vaccines utilizing mRNA are actually safer for patients, more effective, faster and cheaper to make, and don't need to use infectious elements to teach a human body to fight a virus like the old way. It can not and does not change any DNA. It simply teaches certain cells to create an antigen. And Like antivirus software on a computer...you need to update it sometimes
Natural immunity is not better in cases where a sickness can kill you. The vaccine has proven to give you a better chance of survival. A vaccine is giving your body's immune system the best chance to fight off the virus so you don't end up in a hospital.
Your body may feel weak after a vaccine or booster shot because it's kicking your immune system into responding and getting ready to fight a virus. That's why you feel tired, sore, feverish, or may get a rash or swollen lymph nodes. It's actually a very good sign because it means you know it's working.
Hopefully that makes sense.