It was inevitable (and is arguably the "logical" extension of sponsor segments).
As for what it will do to timestamps: The same thing it does to timestamps in podcasts. Some podcast players have a special way to tag the timestamp to adjust with the inserted ads but NOBODY hosts with those. So they are rendered useless.
On the youtube side? They could potentially be auto-adjusted because youtube will know how many ads were inserted . But considering the goal of this is to serve ads...