It's still a step in the right direction, from both moral and practical perspectives, for what is a clear historical wrong. Among other things, it will lay responsibility for the status quo where it really resides, i.e. the US.
I think it's premature to say it would be a step in the right direction, unless Mauritius publicly promises that they would immediately evict the US base (turning down the rent payments and weakening the presence of a strategic ally in the region). From what I've read: "Although Mauritius wishes to gain sovereignty over the Chagos Archipelago, its government, and that of its diplomatic supporter India, favor an American military presence there."[0]
Also, the status of the British Indian Ocean Territory is not a clear historical wrong, or at least it's only wrong due to being a by-product of the original policy of colonialism. Legally speaking, "Council of Ministers [of Mauritius] confirmed agreement to the detachment of Chagos Archipelago"[1], so the objections to this policy are based on the fact that Mauritius was not fully independent from the UK at the time (and therefore under duress), and that a country can't legally be split in two unless both pieces agree (and no one asked the Chagossians specifically if they were happy with the arrangement).
However, given the non-viability of the territory as a self-supporting entity, and assuming that Mauritius would not have wanted to support the unemployed population there after Britain closed the plantations, it's likely that much the same outcome would have happened even if all of our 21st century principles had been followed back in the 1960s. The only difference would have been that the Chagossians wouldn't have received their compensation, reparations, and British citizenship.
