Hi Antoine,
Addressing the point about resources, SegData is designed to move data that would otherwise be in OP_RETURN, witness stuffed, etc. With the segregation, aside from the absolutely necessary tip validation, the segdata is not required either by IBD or storage, and validation is skipped, so node resources are potentially saved in multiple dimensions.
Addressing the other point, as you say anyone can timestamp, so this doesn’t change that in principle. However we can’t force people keep their data off-chain so this covers the case where people insist on putting it on chain, which is what is happening in the wild, but instead of OP_RETURN, creating a structural semantically deliberate region with intent which supports consensual retention.
I hope that answers the initial question. Please continue to press if there’s something i’m missing.