Isco, Modric, Kroos, Verratti, Thiago.
They're not physical beasts, but they are creative, agile and pass the ball forward to create chances. Paulinho might be nice to have, but we definitely need more of those players in our midfield.
All of those above:
1. are technically better than Rakitic
2. are faster, more agile
3. try way more forward passes
So, I agree, it is NOT only about pace and muscles.
But our current midfield trio (Iniesta-Rkitic-Busi) seems happy (or thy can't offer anything else) than: pass the ball left, right and back, or play some little tricks and nice passes 40 meters from the goal and that's it.
As said above, current midfield is good enough WHEN Messi is saving their asses.
But when Messi can't score, we will need some different options.
Those could be some ranged shots from midfielders, some crosses and headers, late runs etc.
Real has:
1. possession game
2. crosses and headers when needed
3. tons of outside of a box shots and goals by Kroos and now Asensio
4. corners and headers
What did we have till today?
1. possession game. The end. Nothing more.
Crosses? No, since all players are short.
Corners? No, since all players are short.
Shots from outside of a box? Zero attempts for Months from Iniesta-Rakitic-Busi.
Late runs into a box? Zero before Paulinho (ok, Roberto offered some of those).
One of the ways to improve a team is to offer a few different options to attack when things aren't going well.
Again, we are spoiled by Pep's era, where our TikiTaka was enough.
But that was working only in a short period of time.
And always before that, our teams had a few different options.
Van Gaal's Barca had possession, crosses and headers due to tall strikers (Rivaldo, Kluivert, Lucho), we had corners and headers and tons of long range shots (Lucho, Figo, Rivaldo, Cocu).
We didn't rely ONLY on possession like today.
In Rijkaard's Barca, the same.
We had Joga Bonita and possession.
But we also had crazy Larsson for corners and crosses.
We had Deco and Van Bommel for crazy long range goals etc.
Pep "killed" all other options since TikiTaka was perfect back then.
But TikiTaka is dead and we need to build a team around 2-3-4 different attacking options.
Real has at least 3-4 attacking options currently.
This is why I am saying that buying the same type of slower, physically weaker, but technically good midfielder over and over won't improve us.
(This is about Seri, for example).