It all wrecks things. Tooth decay will damage teeth as they eat away at the structure. Our jobs as dentists is to minimize the damage we cause when we fix the problem. So...The combination of decay and corrective measures does damage a tooth. The hope is to minimize the damage and hope that it stays like that.
Yes. Dental decay eats away and permanently damages tooth structure making the teeth weaker. The bigger the cavity, the weaker the tooth. Fillings repair the damage but are still not as strong as the original tooth structure. Fillings (or crowns) are necessary to repair the damage. The strongest teeth are the ones that have never had cavities or fillings. Watch what you eat & keep your teeth clean.
Yes. Decay causes permanent damage as well as the tooth structure that is removed in removing the decay. I have seen a reduction in the number of teeth that crack or split when white composite fillings are used instead of the mercury containing silver metal fillings. Metal expands when eating hot food or drinking hot beverages.