And that’s where the idea dies. If we can’t ensure the timestamps are valid, we should not make it part of the design. And we can’t (well, not easily.)
Even if we could, mandatory timestamps simply don’t fit into the philosophy of the network, as @cretz already mentioned. He was not speaking for himself, he just pointed out an obvious fact about how this thing was envisioned.
Another argument: adding timestamps would not make the network do better what it’s design for; another therefore it is unwelcome (EDIT: which, I just noticed, is already mentioned by @cretz; why am I even commenting…)
There are ways to approximate timestamps. For example, there have been talks (sry about shameless self promotion) of building a ledger in the application layer that could be used as an optional (!) registry to store timelines whenever needed.