Currently our midfield is the weakest link however several years down the road, in the post-Messi Barça I think the offense line will be our weakest link. After Suarez and Messi are gone, all we have left is Neymar. Who else do we have in the offensive position? Who else is in the pipeline? Mujica? Munir? Paco?
Real Madrid has Bale, Morata, Vasquez, Arsensio, Mariano and Mayoral and potentially the Norwegian wunderkind all waiting in the pipeline to fill in the shoes after Ronaldo and Benzema are gone, in the offense line alone.
Our firepower will be very underwhelming compared to theirs several years later.
Let's assume that we will have 70-100M to spend each summer, then in 3 summers, that is 200-300 Millions.
Add some money from sales of players whom we don't need like Gomes, Paco, maybe Denis etc, we'll have a nice amount of money.
Till now, we didn't buy a 4th Cf because we wanted to have MSN.
One day when MSN will be gone or when we'll move into a different direction (with only 1 or 2 superstars).
It is hard to BOTH build a team for right now (when we have MSN) and for the future (without MSN).
About your question, whom will we buy for Cfs? Well, currently it may seem that there is no one to buy, but as always, each new season 2-3-4 awesome players start to shine and are chased by big clubs.
Remember the last spring, we all said: if we don't buy Pogba, we are screwed since he will be the ONLY ONE world class Cm and he will rule the world for the next 10 years.
Not only that he is not ruling the world, but also, there are 2-3-4-5 new Cms whom we would like to buy and who are quite good (but who weren't as famous 1 year ago).
So, whom will we buy in 3 years for Cf positions?
1. there will be a few new Argentine kids, Brasilian kids (as always), some random kids from Porto/Benfica, random new gems from French league, random new gems from Germany/Sweden/some Eastern European country.
If we'll manage to buy Verratti one day, we will be perfectly fine.
We will always find new 2 world class Cfs. Maybe not on a MSN level, but we even don't want to have MSN level anymore because they are both a gift and burden for our team.
The question is, who are the potential transfer targets? I don't know any young and promising forwards. Not to mention we will face fierce competitions to bid for the service of any good ones, we might not get what we want.
Or if you are interested, we will quite surely chase 2-3 of these guys in the next 2-3-4 years. One day these guys will be new Hazards, Neymars and Bales:
http://outsideoftheboot.com/2017/01/09/100-best-young-players-2017-9-forwards/
http://outsideoftheboot.com/2017/01/10/100-best-young-players-2017-10-forwards/
The world won't stop after Messi and Cr7.
Both teams will just buy new superstars of that era and we will have new Neymars, Cr7s and Bales. (Not Messis, of course).