I understand the reservations about Myler, but I think its as much about signalling that the time has come for a change in direction as signing him as a player.
We don't have halfbacks of the required quality for next year, but can we afford to wait for one or more of our untried players to make the grade? If they were given a go and didn't make the grade, that would be a third year in a row of 'transition' and a lot of us would be up in arms about why changes hadn't been made. At least signing Myler suggests GH has a sense of urgency about the need to shake things up.
Whilst the Sinfield era was marked by a huge amount of stability across the squad, it was also marked by several major signings along the way. In recent years the stability has remained but we've stopped signing anyone of note. IMO if we didn't start to look externally to fill key positions now, we may end up having to replace large numbers of players en masse, which historically has generally been a disaster.