It is just one common example, not the rule, there are many other situations where this comes into play. What would have happened with Pynchon's post Gravity's Rainbow works if he was not able to get royalties from his previous works during that long writers block he had? During that time his only output was a collection of his early short stories most of which (probably all) were over 20 years old and by the time he got Vineland out all of his work but Gravity's Rainbow would have lost protection with a 20 year copyright. But it paid off in the end and he produced some great works since.
1. Yes, and many never succeed in their life but getting royalties from old works is something to keep you going and a reason not to give up.
2. Those successful works will not be bringing in money for decades to come, just a year or two and then slow to a trickle of royalties. Having a single work which sells well does not mean you are set for life, just means you can breathe easier for awhile.
3. Piracy would not be an issue if that were the case, many already do not pay for it. Sure many say that is because they don't want to support the industry but they are also taking from the writer much of the time.
4. But anyone can now monetize it in all those ways and now the author has to compete for their own work on top of all these extra business dealings and working on new output. And even then things like a bad movie version (which author has little control over) can seriously hurt future sales. This is just trying to turn artist into a brand and not many have pulled that off successfully.
5. This is not really a condition for your point and not even a point.
6. Generally rights are not sold, publishing right are sold to the publisher for a certain number of years and after that time is up the author is free to go make a deal with another publisher or self publish it or what ever they desire.