Well if I own a thing and I leave it alone for a number of months/year without replying after people tried to contact me several time for several months... I can't blame some one to take over on it. Of course if I reply saying:"No!", it's a different thing.
The problem @Chris D is that we are not all devs and there's 100 rows addons but also 6000 rows addons!
a paid add-on is no longer compatibile with 1.5.3... and the author abandoned it. A licensee ask another devs to patch it to support xf 1.5.3 and then make the patch publicly available. it can be a problem without the author consent? (because the author don't respond anymore)
and to add features? (probably with the need to redistribute the whole package)
ok, understood, too much complicated situations... but with many "solo developers" this situations can happen.
p.s. I need to become a developer! (If Chris ever liked this post will be only referred to this row)
If something broke and you posted in the discussion thread a quick fix or you wanted to add a new feature and posted in the discussion thread how to add said feature then I don't think that'd be a problem.
Taking the add-on, patching it/updating it & redistributing it without consent is not permitted.
An add-on, abandoned or not, is still the intellectual property of the person who created it. No-one has the right to take that add-on, make changes and then redistribute it without the explicit consent of the author. This isn't something here on XenForo, it's the case with any software and indeed any other works (text/books, music etc).