I think Swift aims to favour clarity over brevity, and being more explicit in case of oldValue specifically doesn't seem to have any harm.
In the pitch thread, it was also mentioned that this should've been the case since day 1. However, we do have an opportunity to fix that right now if we want to. I would be interested in knowing if people are willing to change their opinion if we also update code completion as a follow-up improvement. I think that once that is fixed then people will feel a bit different about the whole thing because you won't have to write it out manually each time (if you even run into that situation, which is not that common, at least according to my research). Of course, people who think there is a lot of value of having implicit oldValue are not going to be persuaded by changes to tooling.