A few axioms:
- Truth survives. In this context, it would be algorithms and data models - that which truthfully represents the "what" and the "how" of your problem space. Although, there is always the potential for refinement and improvement, or an evolution of the problem itself.
- Languages evolve. This is as true for computer languages as it is for natural languages.
- All technology is vulnerable to obsolescence. It just may take longer for some technologies than others
The most stable technologies and standards (those least vulnerable to obsolescence) tend to be those which are non-proprietary and have been most widely adopted. The wider the adoption, the greater the inertia against almost any form of change. Proprietary "standards" are always vulnerable to the fortunes and whims of their owner and competitive forces.
Twenty years is a very long time in the computer industry. Five years is a more realistic target. In five years' time, the whole problem your application is meant to solve could be completely redefined.
A few examples to illustrate:
C and C++ have been around for a long time. They have implementations on just about every platform. C++ continues to evolve, but "universal" features (those available on all platforms) are pretty much guaranteed to never be deprecated.
Flash almost became a universal standard, but it is proprietary. Corporate decisions to not support it on popular mobile platforms have basically doomed it everywhere - if you're authoring for the web, you want your content available on all platforms; you don't want to miss the major market mobile has become.
WinTel (Windows/x86) despite being proprietary to Microsoft and Intel, having started out on a less-than-optimal platform (16 bit internal / 8 bit external 8088 vs contemporaneous Apple Macintosh 32 bit internal / 16 bit external 68000), and erosion to Apple in the consumer market remains a de facto choice for business platforms. In all that time (25 years), a commitment to backward compatibility has both hobbled future development and inspired considerable confidence that what worked on the old box will still work on the new one.