Let me start with an analogy. Say you got a notice from your bank, “We’re preparing to implement brand-new internet banking software. Controlling your accounts from wherever you are will be easier than ever!”
Then they go on to add, “To save money on this project, we’ve let half our staff of software engineers go, and hired some new ones fresh out of college. They’ll be writing our new implementation. Look forward to the launch in six weeks!”
Would that instill a lot of confidence?
Legislating is like any other job–it takes a while to become competent in the role. A brand-new legislator may know law from the perspective of a lawyer, but that doesn’t mean they know how to write it. Legalese is like a programming language; it doesn’t correspond directly to spoken or written English, and it has a whole bunch of weird, fiddly things that have to be addressed, because with hundreds of millions of users, someone will stumble across all the bugs and unaccounted-for use cases. Novice legislators don’t have the institutional knowledge that says, “We always put a Clause X in bills like this, because they have blown up in our faces in the past when we didn’t.” They also don’t have the familiarity with the structures of government around them to immediately know who to ask about what, which throws more sand in the gears as they try to figure it out (or ask the wrong people, or just give up and wait for someone else to do it).
These factors not only increase the chance that novices will simply make buggy law, they contribute to novices being much more vulnerable to lobbyists who bring them a whole package. “Don’t worry, we’ve done all that research for you. Just back this bill, and it’ll be okay.” (Not to say that many “experienced” legislators don’t opt for that easy path, especially if the proposed bill is delivered in an envelope that another sort of bills happened to fall into, but novices may literally not have any idea what else to do.) The shorter the term limits you impose, the higher proportion of novices you have making laws, and the more of this sort of trouble they’re likely to get into.
Of course, we also have the inverse of this problem–fossilized legislators who have consolidated too much power in Washington and in their home districts, who use their power corruptly and do a lot of harm. Since sheer self-interest makes the other legislators very reluctant to get together and discipline the bad actors, our only current remedy is to try to get them voted out. Term limits would remove these problematic legislators, but they would also cull experienced hands who can teach incoming novices the ropes.
In pragmatic terms, some kind of term limits would probably improve the situation. The problem is balancing them. Too short, and you end up with a disastrously naïve Congress. Too long, and you end up back where we are now, only we’ve handed the experienced ones a new toy to weaponize against their rivals. And since it would require a Constitutional amendment to implement term limits, changing the duration would be extremely difficult if we guess wrong.