Judges deciding cases based on what the law says, free from fear of political reprisal is the intended feature. Lifetime appointments get you that (or very nearly to it). If you know of a strictly better way to get the same feature, what is it?
You might lose some expertise, but on the other hand, you don't have the tragic spectacle of Justices dieing in office whilst waiting for a 'safe' time to retire.
Taking again Germany as my example, since I know it best: There are 16 judges with a period of 12 years. Adding the age limit and other reasons that averages to about 2 elections per year. Thus a bit more than 4 years till a new group can take over the majority. (Now they are elected alternating by the different chambers of parliament, where one of the chambers, Bundesrat, is also slower moving in replacement ... making it hard for a short term movement to take over control and if, impact is relatively short - unless they stay in power long enough to replace all.
Let them serve for a limited period of time, say 16 years max. Can't be removed before then. It's not that complicated and would prevent justices who literally sit on the bench until the day they die.