Without Dembele we lack penetration through the middle. Alli hasn't particularly gotten going yet, and he would benefit from Dembele regardless since they complement each other.
The situation with Dier and Wanyama is the same problem with every other pairing of two defensive midfielders, and arguably a similar problem to what we had when the pairing was Bentaleb and Mason. They're too similar, and their strengths and weaknesses don't complement each other. With Dier-Dembele-Alli we've got a great trinity in midfield. Dier performs the donkey work and works with the defenders, playing simple passes and leaving the holding work to Dembele, who uses his vision and control to dictate pace and transition into attack, while Alli has the freedom to try audacious and creative wonders.
Wanyama-Dier leaves both with conflicting roles in the team that don't help our control through the middle.
