That's exactly it - backwards compatibility. Back at the end of the eighties I was desperately trying to find a way to update documents on-site when I visited customers (and I would sometimes stay for weeks or months), where I couldn't use the word processing system we used at home. I needed something I could run locally, on the laptop (yes we had them) as well as on the target systems (servers). Tried a lot of stuff.. eventually found LaTeX, wrote software to convert our old documents to LaTeX, and a document class which created the same layout as the original (most LaTeX converters "hardcode" the layout [or at least used to, back then] - won't do, as my company changes the "template" now and then).
Everything finally ok. I and colleagues could write documents on-site. A guy from another company had the same problem so I wrote a document class for his company's layout as well.
So, I have these documents from way back, in LaTeX, and occasionally we have to produce them again. Sometimes we re-create them with a newer document class. Other times we extract chapters to be included in newer documents. If LaTeX had broken backwards compatibility it would be disastrous. I won't touch any replacement which doesn't guarantee backwards compatibility. However old the documents are.
