Percentage of Senators with >16 Year’s Tenure:
1819 – 1821: 1%
1915- 1917: 5%
1995-1996: 14%
2017-2019: 20%
The Framers' Constitution naturally limited the semi-permanent senatorial class.
The essential problem with the 17th amendment is that it accomplished one spike in federalism, by changing the nature of the senate from a “representative” body representing the states (and chosen by state legislatures) - acknowledging the states as sovereign entities having their own role in federal decision making - to a body no different than the house - a “democratic” body representing federally voting constituents, and chosen by them.
Instead of the senate truly representing the states, as entities, it became a smaller version of the House, and pushed and pulled by the same monies and interest groups electing members of the House.
On your numbers:
So 80% are not currently “lifers”.
Governments always tend toward corruption. If its collecting taxes someone is skimming, trying to or selling “influence”. No matter how good your founders are or near perfection you set it up to be honest over time it “evolves” toward corruption. It’s the nature of the beast. All you can do is throw sand in those gears. Try to control it, misdirect it, if you’re very clever and lucky misdirect it to a greater purpose but it will always evolve back. Term limits is necessary “sand”, as is repealing the 17th amendment. Easy “sand” when you have executive branch is revoking the EO that allowed federal government unionization as is actual vigorous enforcement of the Hatch Act. So far GOP administrations are too cowardly to do this. (GOP administrations pretend to, Rat administrations don’t even try !)
Radical Amendment Proposal:
Federal employees not only can they not unionize, they give up the right to vote in federal elections. No one forces a person to be a federal employee. Make not voting in federal elections be part of the personal calculus in taking a federal job. I think the Founding Fathers intended that by making the federal district an enclave that didn’t participate in state or federal elections.
I think a similar rule should apply to state, county & city elections. State employee, then banned from voting in state elections, can vote in federal, county & city. County employee then no county but everywhere else, and etc. It would likely require those election to be held at different times, but the gains might out weigh the costs. (Have “voting week” maybe !) However that’s up to those locales.