I don't think Pietsch is necessarily a poor decision maker overall, I think his decision making is poor in the back 3, particularly when it comes to finishing. I think a lot of his play and skill set would actually be well suited to 13 and he has a stable enough base in the 15 man game to make the switch.
As far as Sua'ail'i at 13 goes, I'm not convinced it's his best position, but even if it is that's going to be a long term project which will probably start with a year or 2 almost entirely played in the back 3, largely on the wing to start. The tahs need someone there next year (and cover for them as well) and with Pietsch unlikely to start in the back 3 ahead of Sua'ail'i, Kellaway, Jorgensen and Lancaster (I'd have Reilly above him as pure winger too), there's a lot of upside in him making that change, even if it's just so that he can offer more coverage in the 23 jersey. If they had more centres in the squad it might be a different story, but as it stands they've only retained Foketi and probably Walton with no others rumoured from that Rebels group. The rest of their options are all makeshift who are better in other positions, so you may as well work on converting the "odd man out" in Pietsch. Otherwise he becomes very difficult to re-sign in a years time, where other teams will likely be able to get a lot more value out of him, particularly as Jorgensen comes into his own.
