The TCO of the large X-Y pen plotters seemed subjectively expensive, for hobby purposes, which I suspect is most of the reason to still use a pen plotter. Large paper (maybe you can use the current inkjet ones), expensive pens that you go through quickly (I don't know whether they're made anymore, or whether there's a viable DIY way to keep them refilled and working, or to adapt current off-the-shelf non-plotter pens), and keeping the mechanics of old machines working (and perhaps stockpiling backup/parts units, when you can find them).
The original pens are, AFAICT, somewhat hard to come by, but we modded the pen carousel and have a 3d printed holder to put more modern pens in. With this mod, the TCO doesn't seem to high, although I didn't initially purchase/set it up, and I haven't had to do serious maintenance yet.
But you can throw a pen into any gantry mechanism, whether that's a cheap laser like the k40 or a hobby router like the xcarve, or a kit-built machine like cncrouterparts sells.
These are cheap and ubiquitous. Most of them speak gcode not hpgl, but that's just a firmware tweak or a postprocessor change.
