Excel or Word will randomly fail if the file is old enough. But ok, so Word/Excel can upconvert files if needed. Fair. But does a new document opened in the current Office program look the same? And I mean precisely the same, with fonts and sizes and indentation saved? And what about UI? Don't features appear/disappear?
As for Java - it's mostly compatibile in source and in bytecode:
like with org-mode, the compatibility is broken in some weird cases.
Consider also the file formats. Both Office formats and Java bytecode are binary formats not meant to be read by humans[0]. They can afford backward-compatibility measures. Nobody will bat an eye when new updated version of an opcode will get called CALL2, CALL3, CALL_FIXED, etc. Nor will they care if version flags start showing up in document formatting tags.
Org-mode, on the other hand, is a thin layer of visual formatting over a plain text file. Except in few cases (links, folding), it doesn't change the visible contents of the file compared to opening it as unformatted text! In org-mode, your text file is your UI. It is meant to be edited by hand. So you can't clutter the file with backward-compatibility measures.
Now org-mode provides some elisp scripts to upconvert older documents. They probably don't cover every case, but that's kind of normal - you can't cover everything, and e.g. the cases discussed here today are some pretty advanced / rare ones. Still, it's cheap to manually fix org-mode files - you can just open it and take a look! Try that with Java bytecode or Office formats. You can't - hence the additional effort in ensuring compatibility between versions.
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[0] - Yes, I know new Office format is zipped XML; it's still not meant to be read or modified manually.
Opening an old Excel or Word file does not randomly fail (last time I checked).
Or, if you pick some circa-2005 Java library, it still works.
So it is not clear why org-mode gets a free pass here. At least they could prompt you that "your stuff will not work" if you upgrade.