In general when one looks at the longest serving politicians, it seems to reek much more of extremes of corruption than skill. The insider trading this article references being but one aspect of that. It also correlates with people turning into warmongers, but that's probably just another angle to corruption since there's a rich and ridiculously unaccountable flow of money in military related stuff.

For another datum consider presidents' second term. This is often where they are supposed to be able to really act out their vision for the future since they're ostensibly out of politics after this and have at least 4 years of experience at the highest level, yet they almost invariably tend to be extremely underwhelming.

I'm not arguing there is no skill, but I am arguing that just spending a lot of time in office doesn't seem to correlate much with achieving that skill.