Sounds like we had quite a different experience, and picturing different things too.
> In that case, open source rarely has even one possible replacement, so there's no comparison.
There's usually just one program shipped by a vendor in these cases, and most of the time it's indeed closed-source -- that's what I started with.
Big and widely used FLOSS projects are far from these programs shipped by small hardware manufacturers, I wasn't talking about those. Just as the established and polished commercial projects are far from those: you're getting some buggy and unsupported programs from unknown hardware vendors, possibly even with malware as in TFA, not Photoshop.
> Have you honestly read over an entire open source program to check it?
I have, pretty sure that many others read those too, but haven't read entire sources of large projects like GIMP; plenty of programs and libraries are just a few KLOC (or even just hundreds of LOC) long, easy to skim.
> As to modification, I've reverse engineered many, many programs to add hooks and interoperability. It's not that terribly difficult once you've done a few and get to know how to do it.
I have rather hard time imagining these being any major modifications and considered easy with arbitrary compiled binaries, while suspecting merely reading sources being something mythical. But once again, you're probably picturing a hairy mess of a huge project's source code, and I picture integration tasks like turning a buggy Windows GUI program into a working multi-threaded Linux daemon -- where having source code makes it easier (and I'm certainly reading at least decompiled code when that's an option), as well as making it practical/easier to see what the program is doing.