There are two reasons why you can expect software to be more and more the cause of airliner safety issues:
- software is eating the world
- software is getting more complicated
The first is a long-term trend now. Look under the hood of any automobile from before the 80s: no computer to be found. Look under the hood of any automobile from the past 30 years: computers abound. The reason for this is that many problems are easier to address in software than in hardware. Of course, you go from N hardware problems to some possibly smaller set of possibly simpler hardware problems at the cost of gaining a set of software problems -- but this trade-off usually pays off. In some cases this trade-off enables functionality that would be infeasible to create otherwise.
The second problem is also a long-term trend: CPUs, systems, operating systems, and applications have all tended to get more complex. In embedded systems the trend has been less strongly towards ever-increasing complexity, but even in embedded systems things have gotten more complex.
Whether the problem is less competence among today's programmers is hard to establish here. First, we need much more software, which means we need many more programmers, which means the quality of programmers you get probably does decrease, though then again, we do have more programmers overall as more people (competent and otherwise) are attracted to the industry. But more importantly, the increase in complexity of today's systems could very well be enough to make yesteryear's competent programmers incompetent today -- you can't really compare software development 40 years ago to software development today.
(I object to this idea that lower-wage programmers necessarily can't be competent, though that isn't quite what you wrote. It's true that a lax process for outsourcing can mean you get less competent programmers, and it's probably true that higher GDP/capita correlates with availability of competent programmers. But it doesn't follow that there are no competent lower-wage programmers in India, say.)