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).