Of course, an argument can be made that the original inventor should be recompensed, and that's fair...to a point. To redress this perceived injustice: patents should expire after a reasonable time, and NOT be renewable beyond a limited extent (say one or two renewals).
Patents do expire after (what Congress has decided is) a reasonable time: twenty years from the date of filing of the application. They are not renewable, period. And they can even expire earlier, if you fail to pay maintenance fees.
Wow, I wasn't aware of the requirement for maintenance fees, very interesting. So essentially, these fees do pretty much as I described, with the exception that the last 'maintenance fee' paid in year 12 extends the patent eight years, instead of only four years for the previous 'maintenance' fees.
Am I correct in assuming that once it expires (for whatever reason), it then gets assigned to the public domain?
Mostly yes - slight change to what you described was that maintenance fees are based on filing date, so if you have a patent application pending for 10 years, you only pay the year 12 fee, and it expires 8 years later.
And yes, once it expires, it's public domain forever. Interestingly, though unsurprisingly, most software-related patents get abandoned without paying the 12 year fee. The fees are not static, but increase significantly, so for most companies, it's not worth it to pay that last extension, particularly in software where technology moves so quickly. So, allowing for the time from filing to grant, most software-related patents already only last around 4-5 years.
Ah, so let the patent term start from the day it's granted, not the day it's filed (I think that's how it works now, isn't it?) That seems fair to everyone. The key takeaway is that patent terms should be drastically shortened, so as not to stifle the very innovations they purport to protect...but OTOH, it doesn't look like the 'maintenance fees' implemented in 1980 are having much of an impact on the patent circus, either.
That is how it works - term starts from day of grant and lasts until it expires, either for non-payment of maintenance fees or the end of 20 years from the date of filing. Now, whether 20 years is too long is open to debate, but it's not like the lifetime+70 years of copyright. And the maintenance fees do have a significant impact - see above - although maybe they should be either increased or have an added increment or two... I agree, having the final fee at 12 years is odd... maybe there should be additional fees due at 15 years and 18 years. That would tend to make patents expire sooner, unless the patent owner really thinks they're valuable.