If politicians are good at their jobs, they may get elected to the next level of political office. For example, a council member may run for mayor or for a job in the State government. A State legislator may run for governor or for Congress. Not all elected people want to advance and many do not try to. Others do not get reelected or just leave the occupation.
Most politicians serve their communities for only short periods of time.
The future The number of chief executives and legislators will not change much. Few, if any, new governments are likely to start up. Some cities and towns may take on professional managers or move from volunteer to paid career executives. In large places, most elections for public office are expensive. This most likely will not change.