There is a very simple argument not needing the avalanche of words you seem to like: A weld must be designed such that it does not depend on near perfect welding technique not to break. It is a safety critical item, so must be over-designed. If designed like that, even a dodgy weld will not result in failure. The hinge plate can be designed to clip into the frame that it would not fail even in the total absence of a weld. I published a sketch of this idea in this forum. In Terns case as well as the ill-fated Origin8 bikes, that was not the case. A slightly weak weld led to failure. Now tell me that is not bad design of a safety critical item. Case closed.
Jur, I appreciate your opinion and you have a valid point here. It took Brompton more than 20 years to get there (with the new hinges on the MK4 in 2004 and that was as far as I know by accident, not on purpose but as a collateral effect of a new machine for brazing in the hinges more efficiently). Before that braking frames were a wellknown issue with Bromptons. Just w/o any dramatic crashes as they are made of steel and therefor break in a more userfriendly way...
Anyway I think your point is valid - someone designing a bicycle has to find the right balance of weight, complexity, production cost, optics, functionality, robustness and tolerance for failure. Tern obviously seems to have underestimated the necessary amount of tolerance for failure or lacked quality management in the factory (that possibly could have compensated that).
I'll leave out the bolt issue and focus on the weld topic to make things easier. No doubt your proposed design would have made things more tolerant for failure. But on the other hand aluminum frames are the standard on most bikes today and joint welding (not sure if this is the correct word in english) is the standard as well as it is cheaper and offers more flexibility in the design. Only rarely we see issues on bikes welded that way. So in general it seems to be kind of ok - but maybe not on the hinge of a folder?
Hmm, there are many folders out there with aluminium frames and a hinge on the main frame. Rarely there are issues with the weld in this area. Are they all made the way you proposed? I do not know but I doubt it.
Even better: When looking at Tern one can see that not all variants of a model had issues. It were mainly the (cheaper) D/P 7/8 models of the Link series (the mass-models) and the lighter models of the Verge and Eclipse series if I remember correctly. There were (as to my knowledge) i.e. absolutely no issues just on the touring models like the Link P24 or the Verge tour despite one can assume those would be ridden more intensively and with more weight. I do not know if the hinge follows the same design with those but would assume that it is. Maybe it is welded slighly differently - I don't know. And there are other models that were not affected as well. This is one of the reasons why I buy from Tern the story of a single factory producing bad welds for a limited amount of time at least as a possibility. And why I would not say it is a generally bad design. No doubt it is not the optimal design in terms of failure tolerance and could be enhanced in one way or another (or changed completely) but it seems to be more an issue of craftsmanship and quality control than a faulty design. In other words: If you work thoroughly you may i.e. safe on weight or cost by reducing the tolerance for failure. Therefor Tern was possibly "just" to ambitious, making wrong decisions and overestimating what they (or their outsourced factories) would be able to do and to achieve while putting not enough effort in quality control. And possibly they still didn't over the last years as there are still frames snapping. Probably not as often as with the early models but - as said before - one single incident can be one too much.
As to your design: As you describe it it would stop the bike from suddenly breaking apart (good). On the other hand it only cures a symptom (breaking apart) but not the root cause (bad weld). And you can assume that people would still use a bike with a broken weld ignorantly on a daily basis just because it still works - who cares for a broken weld anyway... Not sure if that's what you wanted to achieve.
Nevertheless it would be an enhancement that would relax riders in much the same way as it was achieved by the addtional weld on the younger Verges (possibly in a cheaper way).