My main problem with it is that the England coaches do not have to live with any negative consequences of moving a player. If Slade moves to 12, Exe will have little back-up at 10 and would have to move Hill presumably to 13. It would massively change their 10-12-13 dynamic. Even if it goes well for Slade and he shines, the other players might not. And if Slade is no good there, England simply don't have to pick him. There is absolutely zero accountability towards the decision-making if it were to be unilateral. It's not in the interests of the clubs, players or fans, and has a fairly marginal effect on England.
I like the current system because England does have power up to a certain point which allows them to effectively move the key players, but if what they are suggesting is madness, then the clubs have a means of stopping it. Like Burgess at 12.
Slade hasn't moved to 12 because England haven't been interested in him moving there. Things might be different if they were.