I think it is possible to convert a bridge into a tunnel, to follow with your extreme example. It's totally possible. The only thing is that when the cost esti

I think it is possible to convert a bridge into a tunnel, to follow with your extreme example. It's totally possible. The only thing is that when the cost estimation arrives to the hands of decision makers, they open their eyes wide and decide that for that money they better go ahead with a bridge. So IMHO it's just a matter of cost. Say you go with a bridge but there are some initial wooden stairs, and sometime along the middle of placing them they decide it is acceptable to pay the cost of stopping and replacing them with metallic ones. Could happen. Biggest change I experienced in the rate of unexpected last-time feature changes in software, was having a project manager that first of all knew how to say "No" (or discuss the sense of the requests), and secondly, was able to communicate the real cost that each change would have (in terms of developer time, which at the end of the day means money). Then it would be the company's boss himself who would decide to discard the most crazy changes that were "mandatory" and coming from the sales people.