If the forecast or the traffic holding requires x amount of fuel and you have put that on board then there is no problem. If you only carried x and the requirement is now x+y then you need to have x+y fuel on board. If you don't have x+y then you go find somewhere to land that doesn't require x+y and put more fuel on. If you continue to your destination and ignore the new requirement and end up landing with 10 minutes of fuel in your tank then that is a decision that you will have to justify. If you have decided to arrive at your destination with only the FR and your destination is now closed due to a disabled aircraft then you are required to declare MAYDAY FUEL and do whatever is required to ensure the safety of yourself and your passengers. If you had a possible alternate that is 10 minutes away you still have to declare a mayday if you will arrive with less than 45 minutes. If you stooged around waiting because ATC or whoever said it will be cleared up in 35 minutes then had to divert and arrived with only fuel vapours then once again you would have to justify that decision.