given that people choose to work on core technologies for free, no company is investing in those technologies. The underinvestment means that the core technologies are often unfinished, lacking quality, have a lot of rough edges, bugs, etc. That, in turn, creates need for duct tape and thus proliferation of duct-taping jobs
What's so wrong about duct-taping jobs? There's nothing wrong with building upon software that achieves much of the functionality needed for the application; there's nothing wrong with further developing software that's not complete for your subjective time-frame. It's not sensible that "the perfect application software" inherently exists for your subjective application; it is only by mere circumstance that "the perfect application software" happens to exist for your subjective time-frame all without any input on your part.
software engineering work was divided between the interesting and challenging work of developing core technologies, and the tedious labor of “applying duct tape” to allow different core technologies to work together, because the designers had never bothered to think about their compatibility.
the less incentive they have to make them compatible with other such software
I have no idea what he means by this. Why does any programmer need to design and develop "compatible core technology" on an inherent basis? It is not sensible for software technologies to be compatible on an inherent basis. The reason why software is compatible is because people intentionally and specifically design the technologies to work together. This implies that there is a predefined and prearranged standard for the software technology to work together; I have no idea why he would think the such compatibility standards would inherently exist for all software.
His main point, though, was that, increasingly, open source means that all the really engaging tasks are done for free:
I already stated that duct-taping jobs for software development has no negative meaning for me. For the case of software development, duct-taping jobs are perfectly normal because the alternatives are the "magical and already perfect software application with zero input on your own part" and "software that doesn't exist in any capacity".
in fact people these days complain about a corporate takeover of Free Software.
The big thing that's important to me is whether the software respects my freedom as the user. Matters of corporations being the major sponsor of development has zero relevance to me. I do not care that there are highly complex free software projects that exist. I believe that having freedom implies taking personal responsibility to invest your own resources.