I don’t necessarily think the founders fucked up. It’s important that the court be free from political influence when deciding cases so I think they had the right idea. I’m not necessarily opposed to lifetime appointments. Where I think there’s a lot of room for improvement is the nomination and confirmation process. It’s entirely political, contentious, and has produced a few lousy justices in recent years.
This idea of one party only appointing conservatives and the other only appointing liberals and both sides hating the other’s appointments is what’s fucked up. What could be interesting is a bipartisan Congressional nominating committee that produces candidates that are at least palatable to both sides. Let’s say there’s a 2/3 majority requirement for the committee to nominate someone. They could produce a list of several candidates and the president nominates one of them. Basically take this process away from partisan NGOs and give it to a bipartisan group of elected representatives.