I don't think it's a good thing. In fact, I think the WWE have so many more options and possibilities available to them if Rusev wins and beats Cena cleanly.
For example, Rusev beats John Cena at Wrestlemania and no matter who wins between Lesnar or Reigns (would prefer Lesnar to) have Seth Rollins cash in and becomes the new WWE Champion to close Wrestlemania.
Roman Reigns come out the next night on Raw after Wrestlemania and Rusev comes out to gloat. A match is set up for later in the night for the US championship and Reigns defeats Rusev for the US championship. Gets the crowd behind Reigns and gives him a chance for the crowd to connect with him as a champion while also taking the US championship away from Rusev and ending a streak before it becomes career defining.
With John Cena losing, you go on a redemption story. Its loss after loss for the face of the company. Time is catching up to Cena so he goes after WWE Champion Seth Rollins. Have a triple threat match with Orton, number one contender matches until eventually the face of the company becomes once again WWE Champion. John Cena becomes a 16 time World Champion.
If Lesnar is with WWE at this point, have him feud with Rollins.
Rusev, who has been on a rampage since losing the title, beating everyone but while not going after the US Title - 'reason - why do I want to represent you stupid Americans' - demands a match against WWE Champion John Cena, pointing to two straight victories. John Cena accepts and it's WWE Champion John Cena against the unstoppable Russian Rusev at Summerslam. Rusev wins, becomes WWE Champion.
Who can stop him?
You have three choices at Wrestlemania: Reigns if his US Championship experiment worked, Brock Lesnar (if he stays with WWE) or the Rock.
Then you've elevated Rusev and maybe Reigns by giving him an acceptable title reign, have a chance to make Brock Lesnar USA's saviour, keep building Rollins as the architect heel, redeem Cena while passing the torch and maybe even if it's played correctly get that Rock vs Brock match (title or no title).
By the way, I haven't mentioned Daniel Bryan as I want to see him as Intercontinental Champion and hold the belt until next Wrestlemania, bringing real prestige back to the title.
So, in conclusion, no I wouldn't have Cena beat Rusev as there's a bigger story available for both characters by having Rusev beat Cena.