Deprecating the Formal Public Identifier was probably a good move, but using a web URI is nearly as bad. If the GCA had realised what they had in the ISO 9070 Registry, they could have made a big difference. Formal ownership of Names is important.
psgml risks being lost because of the introduction of nxml-mode, which handles only RNG, and has a very limited control set, making it virtually unusable as a text-document editor
Although the family-tree hierarchical box diagram representation of the schema or DTD tree is common in many XML editors, none of them yet appears to match the design, clarity, and ease of use of Near&Far Designer
The use of XML in the web browser has never properly been supported by the browser-makers, for the exact same reasons as the original HTML wasn’t. It has to some extent been saved by Saxon/CE and Saxon/JS, but the browsers themselves are a lost cause
Open Source software (then usually just called free) is no better at surviving three decades than commercial software. In fact, commercial software may have the edge, in that it came in boxes, with manuals, CDs, dongles, licence keys, and other stuff, so it got put on a shelf or into a cupboard.
However, most (but not all) web sites acting as repositories for the free software have long since disappeared; but so have almost all of the corporate web sites of the commercial software.
What is particularly pernicious is that when the owner[s] of a popular and much-used commercial product diversify (or, sadly, die), and it is then sold to another company, the buyers usually knows roughly what they have bought — the first time it happens. But when those buyers are themselves bought, the third owner has less knowledge and interest. By the time it happens again, and maybe again, a once-reputable product is now owned by a manufacturer of children’s toys, and has no idea why it also sells a structured-document editor. Let the market decide only works in the textbook economic circumstances of perfect knowledge. In the Real World™ where everything is kept under wraps, nothing is safe.
Hypertext linking in the HyTime sense never took off. The Panorama-style browsers demonstrated that it was not only possible but easy to use, and anyone who has taught HTML or had to deal with novice designers will know that there is demand for multi-headed and bidirectional links.