Doing DB operations is relatively simple, formdata entry and validation is super easy, supports modules, OOP and even compiles to native code.
Yeah not good for networked operations, but in many mom and pop shops it isn't needed anyway.
Even VB/Delphi for all their RAD capabilities were already much more complex than Clipper, but they messed up their migration into the GUI world and CA Visual Objects was too complex for the Clipper crowd.
COBOL/FORTRAN/Prolog/etc type applications are out there, apparently, and if you're in the right place at the right time you can entrench reasonably effectively, at least short term.
However, these types of jobs also have their own unique pain points. This recent article (they surface reasonably frequently on here), and the comments, make for good reading: https://news.ycombinator.com/item?id=25148840
As for PHP+et al, I recommend these for their overall simplicity, relatively low learning curve, and ability to scale "well enough", for above-average values of "well enough". PHP itself isn't the most ideally designed language, but it can actually get things done. Furthermore, its reputation as a "simple" or "stupid" language, like eg VB6, can also serve to catalyze the upper bound of expected complexity of a particular solution. This has the downside of maybe a bit of isolation from truly interesting challenges, but allows for a slower/relaxed and perhaps more maintainable pace. (And there's nothing stopping you building something Facebook-sized if you needed to.)
On the client side, HTML5 is expansive and... very backward compatible with how things have been done for the past 20 years. Elitist forum commentators may make snooty noises about your use of tables, but Chrome won't. JavaScript could be a harder language to reason about, especially for simple enhancements like fetching bits of data which won't require learning pages of theory first.
Finally, my recommendation to focus on the Web is that, well, once you grumble and context-switch to "ugh, fine, HTML+JS", everything else can roll in incrementally as you go along.
With something stuck in the 80s/90s, sure, you guarantee that you're completely isolated from the firehose... at the cost of being objectively less economically viable in industry.
At least here, the businesses that still ran xBase were the ones unwilling or unable to pay decently for maintenance and/or migration, and that was the situation 10+ years ago.
Haven't looked into Harbour featureset, but I feel xBase in general is simply too simple for new developments. Even mom-and-pop stores need features way beyond the Clipper/MS-DOS capabilities.
The language is outdated, too; even Delphi/Lazarus sounds too 1990ish to me, I have fond memories of Delphi and Object Pascal but it is impossible to justify using them in a world with so many good new languages.
Clipper/xBase systems that do their job is one niche of MS-DOS apps that refuse to die. Another niche I know of, is gambling machines (one-armed bandits), for some reason people kept using DOS (at least they moved to FreeDOS by early 2000s).