Even if a decline starts around that age (and this is highly player-dependent) it doesn't mean the player can not compete at a high level for several more seasons. So I'm sure Poch would have no issues with having a 29-year old Sabitzer in his squad.
He might not but adding someone like him is not a piece for a rebuilding squad. He is an immediate player that will likely have diminishing returns by the time Poch returns to the club.
If we are looking to add players for a rebuilding squad for Poch in a few years we should look for players that will be hitting their peak at that time.
Either way I don't have much interest in Sabitzer now or for the future.