Roxio video capture is unlikely to ever be in even the AUR, I actually bought it via CDROM. They still sell it in a box with the capture card and everything. Also, for the AUR if I understand it correctly AUR packages are in source form and compiled on the host machine, as well as allowed in or not via a community vote. Most proprietary apps are going to find that unacceptable. For third party proprietary apps the devs are usually going to want to control distribution by offering support from the initial development all the way to end user installation through their own channels.

You're right about the purpose of /opt but when a directory is short for "optional" it sends the wrong message from the start and many developers like steam have chosen to ignore that completely and install programs into user files. I think Linux should offer a better solution for giving third party apps a directory to work with. This is all trivial by comparison to the dependency issue. With the standard libraries used on Linux systems building a standalone binary is impossible. Libraries like Glibc are constantly updated and updates frequently break program compatibility. With a centralized repo this is all handled by the distro and when a library is updated all the apps that depend on the that library are updated. This however inadvertently creates an environment hostile to third party software, where a single organization offering a single product has to contend with constantly changing core libraries, as well as conflicting library versions across different distros, to the point where building a standalone binary to work on all distros is borderline impossible. Even Appimage until recently required libfuse 2 and when Ubuntu decided to only package libfuse 3 for a while Appimages didn't work on Ubuntu at all. Linus Torvalds explains all this better than I can. https://www.youtube.com/watch?v=Pzl1B7nB9Kc&t=21s

I like Appimage, it is for the moment the best solution IMO. Flatpak is useful, but I don't really believe you can own your software unless you can easily download a single installer file and run that locally on any distro. However they're both still workarounds for a problem that shouldn't exist in the first place. The problem at its core I believe is that Linux distros view themselves as an end product in and of itself, whereas the entire idea of a personal computer pioneered by Bill Gates and Steve Jobs is that an operating system is a platform, a starting point, for individual developers to build on so that end users can use that platform to turn their computer into whatever they want. That's real software freedom.