If (example) we were 1 left back away from being contenders with an empty DP slot, than hell yeah you fill that ***** up and you go get the best left back you can find. Oyongo is versatile on the field, can play both wing back positions and has the best 1st touch on the team, if a DP tag is what is needed to keep him, I would give it to him. The same goes with Ciman obviously.
I get what you're saying, but you never wake up saying "You know what, we have a good team, but we're one right back away from winning the league" You win championships with great spines: Striker, Center Mids (Offensive/Defensive), Center Backs and Goalkeepers. The rest are what I call complimentary positions. Although they're very important to a teams functionality, they aren't as crucial. You can get away with having "solid" players at these positions and winning the championship.
Second reason is because full backs are the weakest position in world football. Its very difficult to have a world class fullback nowadays, and its very normal to have steady/stable fullbacks a la Toia. What you need to do is evaluate, will having a DP RB like Oyongo improve you significantly more than having a stable one? I dont think the investment matches the reward in such position.
And anyways, as most people said, he wants to go to Europe