It's misleading to describe him as 'free'. He's demand a huge wage and a three-year (at least) contract. It's exactly the kind of signing we have hopefully learned our lesson over. Lots of posters here who will criticise signings of Casemiro, Varane, Eriksen etc yet seem to want this.
There are plenty of young little-known players we could gamble on for £10m or so on low wages and end up spending the same amount we would on Rabiot's wages and agent fee. No real risk, decent chance of resale value even if they're not a great success. One fewer big egos in the dressing room.