The key for me is Rakitic. The whole "MSN freedom" argument for him not playing well would be more valid to me if Rakitic had struggled as much as Arda has. It's been different for him since the ban and I think it'd be rash to sell him this summer. But it's not going to be any easier for him with Denis Suarez and Rafinha needing minutes to develop, assuming no big name MF like Pogba enters the picture.
Playing in Barça's midfield requires not only technical ability but a tactical nous that very few players possess. That's why I'll always prefer home-bred MFs to imports because they have been bred with the specific skills and mental training we need that makes the adaption process much easier.
On top of that, given the freedom Lucho gives to MSN, not only do you have to have all that ^ but now, you have to play fairly defensively and behind the ball. Iniesta in particular had a hard time adapting to this, he always played ahead of the ball, more forward than Xavi and Busi to link with the front three under Pep's era and afterwords. Lucho plays his MF deeper than Pep did in comparison and only recently had Iniesta finally looked like himself under Lucho.
The defensive freedom Lucho gives to MSN will not change nor should it. Any MF who plays here has to adapt to that and if Arda cannot, he shouldn't play until he can. For me, he still looks a bit lost positiontionally, too wide and he doesn't tuck in to help start the play from out deep. He isn't comfortable doing that so his best use to us is strictly as a winger/forward.
A big problem recently is how frequently Messi and Neymar are drifting inside. That's due to fatigue, you have to run more to create from the wings but it is crucially blunting our attack. Every team we have played has flooded the middle and with Neymar and Messi in the center, it's simply too congested to play through. Having those two out wide stretches their lines and opens up space and passing lanes to play around. Believe it or not, they would be having more success if they stayed wide and waited until in possession to cut in.
The best solution to this tactically then is to go into a double pivot with Messi as a CAM and Arda as a RW but then the Don gets benched.
Lastly, I'm not convinced Pogba would do much better than Arda for reasons I've already laid out. He loves to bomb forward and receive the ball in advanced positions and that would put a lot of pressure on Busi to cover the space he leaves.