The other issue is about style of play. Wasps have in recent years, liked a "Jimmy-Style" 12 to provide additional distribution but in the past have had 12's like Hendo who provided a different skillset. Ditto Mark Denny. That was more the AB style with pace and power around the back three (Lewsey, Logan, Sampson and others)
Getting Mills was an indication, to me, that we wanted that "second 5/8th" at 12 to bring hard running players outside, like Fek and Odogwu into play and facilitate the speed out wider
I think we got Booj to provide the contrast at 12 - but not as a regular first choice, at first anyway, and due to his durability, he's been forced to take on more this and especially last season that probably expected. And mainly done v well.
But when that changing midfield is outside a young 10 and with changing 15's (who could provide that second 5/8th meme) it's been very hard. I rather fancy lining up with Dan, Umaga/Mills.Fek and Odogwu, Bassett and Minozzi to provide some contrast