Average? Taking a semi-educated guess I would say maybe two and a half terms or sixteen years but that may be high. Reelection for incumbents in the House? It's extremely high. So your contention is that there are members of the House and Senate that are perpetrating some grand 20-30 year conspiracy to erode the rights of American citizens? Most of them are more concerned with lining their own pockets with graft than they are shaping some new world order. Also, the Senate and House has only been majority Republican for the last ten years or so. Prior to that the Democrats held the edge in both chambers so that tempers the ability to enact any long-term scheme.
Prior to the last two replacements the "Rehnquist Court" had been there for around twenty years. Average span for a Supreme Court Justice is around ten to fifteen years I would say. The longest one was around thirty-ish. While I certainly agree a judge comes into the job with an idea of how the Constitution should be interpreted (it's the main reason I generally vote Republican), the Judiciary branch doesn't execute the laws that is, of course, the job of the Executive branch.
Over the last twenty+ years I would say the biggest change to this nation that has come out of the Supreme Court has been a curbing of the power of Congress to cram the basis for their Constitutional power to enact laws under the Commerce Clause.