Yep, if club is regular CL title challenger, it may take around 5 seasons to lose enough rating. But nobody deserves such present. On early years it was possible, but those were rare cases, when new manager was able to continue earlier success for more than 5 seasons. Mostly it ended like AC Subotica-Rossoneri story.
With very top and abandoned clubs currently is quite high risk to get into financial troubles as previous owner maybe left team because of bankrupt: having expensive facilities and high-waged players. Just like your previous club.