Want to be clear: I do not think that Barca currently wants Koeman. If I were Barca I wouldn't want him right now. What I mean is that if Koeman is leaving by choice before his contract is up (once he signs for us), he is likely doing it for a massive job (i.e. Barca or other massive job) and he is likely getting that offer because he's done something special here (i.e. winning Europa, PL title, or consistent CL performance).
Koeman will deffo see us as a stepping stone, the thing is though, if one of Europe's elite come in for him after a few years here, and as you say, it will mean Koeman has done a boss job with us.