It is easy to be distracted by images of buildings in their decayed state from causes other than nature and design or material failure. I too find the romantic fascination with ruins problematic.
Whilst the effects of nature may lead to decay the more likely cause of these buildings leading to such outcomes relates more to the decline of certain industries, broken hereditary lines and dilution of royal blood lines, erosion of religious or political beliefs, fracturing of a society through war, large scale disease or starvation, and decline in the traditional use of buildings and space (the decay of the formal dining room over the past century and the decay of the study over the past two decades are good examples of spatial decay).
Also consider the growth in the number family breakdowns, the modern regard by financial institutions of buildings as pure property value as opposed to cultural or architectural value, growth in the complexity of property ownership (successors to title, contested wills etc) which can ultimately lead to legal stalemates spanning generations where the only outcome is the ultimate decay of the buildings and property being contested.
I do not only see the quaint romanticism of buildings in decay. There is almost always a sadness, a loss, a tragedy, reasons associated with defeat, greed, disregard and even disrespect in the value of human creation when buildings are left to fall into decay.
Whilst on an expedition to South America Joern Utzon famously phoned his engineer and said with great excitement that “the Opera House will be beautiful when it is a ruin”. This is the only Architect I know with the vision you are raising awareness of in your discussion.
Perhaps rather than considering how their buildings might decay Architects should be more focussed on how their buildings will be disassembled or demolished for re-use.