Why is it taking so excessively long for Viagra to become a generic drug?

Why is it taking so excessively long for Viagra to become a generic drug?
I believe that with any drug, the company that did all the research and development (usually 10 or so years) to make the drug wants to recoup their investment. Thus they have a patent for a lengthy period of time so that only that one company can manufacture the drug. Once the patent expires then one of two things happens: they can revise the drug and get another patent, or release the drug so that any company can make it. Since all the research and development is already done, those companies can make it much cheaper - they just can't make or market it by its original name (in this case Viagra) - it has to be marketed under its generic name. When you see the symbol "R" enclosed in a circle after the name, that is an indication that is the original trade name. Generics do not have that symbol.