Give examples (other than slavery) of how it is wrong to accept something just because that is how things have always been done. (See below)
A long habit of not thinking a thing wrong, gives it a superficial appearance of being right, and raises at first a formidable outcry in defence of custom.
When a society has been doing things in one way for a long period of time, people come to believe, without having to think about it, that that way of doing things is morally right. They will tend to defend it simply because it is traditional, because that is how things have always been done.
You mention that we should not use slavery to answer this, but I would point out that the closely related issue of racial discrimination is another example of this. White people in America, as late as the 1960s, still simply assumed that non-white people were inferior and should be segregated. This was clearly a case of something that was wrong being accepted because it was traditional.
If we look at the present day, this is a more controversial idea because many people will still believe that the tradition they are upholding is right. One possible example of this would be ideas about gay people. At present, gays are denied some legal protections and rights, such as the right to marry. This is largely because we have traditionally believed that homosexuality is wrong. Future generations may well look back at our times and conclude that we were stubbornly holding on to a wrong belief simply because it was traditional.