I wonder if you are contradicting yourself Gareth.
1) "I disagree that simple themes are less likely to break."
2) "It is true that the more complex software is, the more likely that a fault will occur"
For quite a few people the issue is one of risk. Simpler software presents less risk, that's all I was saying.
In terms of GPLv3 and "you have the source code such that you are in a position to do something about it." that's an easy statement to make, but many people won't have the technical skills to do anything about it. Again it's about taking a view on the risk.
Fair enough Graham, some things are not clear cut and dried.
With GPLv3, at least with the source code you are in a position to do something about it was pertaining to the situation where the code is closed source and you can't. If I have a 'mind that bus, what bus, splat' moment, at least you still have Essential and be able to ask other technically minded persons to take it on. So you 'can' do something about it even if you are not the person at a technical level.