I understand the point you're making. FWIW I think he's had a few dodgy moments this season and, despite some really good form of late, I think he was perhaps better overall last season.
It's a difficult one because his size, speed, nationality and being left footed are all things that make him a rare commodity and we are right to place a hefty valuation on him because of it. However, you can understand why your top clubs would baulk at £70m or whatever it is we've priced him at. It's a huge fee for a player who's not really been in the England thinking much and is playing for a lower Premier League side.
In the normal course of events, he'd be looking at a £40m-ish transfer to a top 7 side before a bigger move, but obviously we see his value now.
With PSR in place for another year, there's still a strong possibility he leaves I would guess given our need to generate balance sheet profits, but I'm not sure a centre back pairing of Tarkowski and O'Brien (and I agree O'Brien looks very promising) has the same balance about it as one which has Branthwaite and that natural left-sided balance.