Interesting post, well researched, and definitely a worthy topic for discussion.
Even if we are to accept your (arguably narrow) definition of a contender, I think one factor you overlook is that even when teams are in a "rebuilding" phase, there will be a handful of years where they aren't quite a contender yet, but have the exciting pieces in place and are clearly moving towards being one. They may never get there, but those years are not "lost" years.
A great example would be the 2-3 years leading up to the Thunder's first trip to the WCF back in 2010. 2008 and 2009 were tough years for the Thunder in terms of winning, but they had their key players in place, and they were exciting.
So while the "10-13 years" for some of these teams sounds especially long, you might have to subtract 3-4 years in some cases because even when those teams weren't contenders, they had their young star in place and were clearly on their way.
The Cavs, as another example, were not a contender until 2005-2006, but as soon as they had LeBron on board they had arrived as a team worth paying attention to.