Hi, most people have the wisdom teeth removed as tend to cram other teeth as we age. Not a big deal as you can have all 4 taken out at the same setting.
Wisdom teeth are so far in the back of the mouth that they are difficult to brush, floss and do dental work on. Doesn't mean it's impossible, just more difficult. Generally we don't need these teeth and they just cause problems, which is why our modern society believes in extracting them. Sometimes, I've heard, if you lose another molar, the wisdom tooth can migrate into it's spot and take its place. But you'll have to ask the dentist is this is possible or if there will be any problems if you choose to have your 1st molar extracted.
Treating a cavity all depends on how deep the cavity has gone in your teeth.
Only in extreme cases will a dentist recommended a tooth extraction. The tooth is completely removed if the cavity is very bad and the tooth can't be saved.
When the tooth is severely damaged by the cavity, a root canal removes the pulp and nerve and seals the tooth. This completely prevents any further decay.