Essentially they come down to one reason: poor decision-making by the current regime at Charlton. As this article in The Wharf says it will be 'an avoidable and costly' relegation: Reasons for relegation
It will also be a difficult relegation to recover from given the widespread dissatisfaction among Charlton customers. Contrast feelings with Northampton where they are so overjoyed at getting into League One they have already sold over a thousand season tickets.
I don't think that promotion will come easily, even if there was a change of ownership. Equally, I do not agree with those who see another relegation into League Two: I think a md-table side is more likely.
However, if the club was relegated again, complete closure might be on the agenda as it would be difficult to sustain an expensive infrastructure. I don't think The Valley could be easily developed for housing because of the demolition costs and the presence of the outfall sewer.