I disagree and this is why.
Sio is only 29, whilst Slipper is top seed atm, he will be 34 at the next WC, so a coin flip whether he will be an epic bench man or retired. See how he goes.
Bell is only 20, so we don't know yet whether he will develop enough to overtake Sio, because he definitely hasn't yet.
HJH (Harry Johnson-Holmes), Orr and Robertson aren't in the conversation. There is no other 19 or 20 year old running about who could develop fast enough in time to replace him. Only other threat is Eloff who has only just started to show something at 22 and again, no idea where he will get to and currently not yet matching Sio.
Point being, at this stage he is a lock in for the Wallabies 23 and has a better chance then not of being in the 2023 WC 23.