The idea of software being patentable is actually a very new concept. Though there are some rare examples of software systems being patented back in the 1980s, it wasn't until the late 1990s that software patentablity was a widely accepted concept, prior to then software was seen as a mathematical construct and thus explicitly excluded from patents. The industry got along fine back then on copyright laws alone and will get along fine with or without patents in the future.
Any time MegaCorp 1 accuses MegaCorp 2 of stealing its stuff, it's going to be expensive to resolve. Even if you draw the line at "copyright, not patent" MegaCorp 2 is just going to push the line that much further in how closely they can copy MegaCorp 1.
Copyright violations require a much more willful sort of of idea "theft".
Or so the pro-software patent argument goes. In practice, there have been lots of undocumented binary formats parts of which are patented. The old .doc/.xls/.ppt formats had that status for years.
"But crucially, copyright protection allows someone to independently develop software to achieve the "same overall result" as a copyrighted program. In contrast, patent law doesn't allow independent invention limiting the opportunities of future innovators and creating the risk of accidental infringement and wasteful litigation."