I love an young up and comer myself, but it's rare for them to come into a full works team. Assuming Alonso didn't come back who would have got the seat? Probably Perez or Hulkenberg returning. None of Renault's junior drivers have shown enough to warrant jumping straight into the seat and having an underachieving Ocon and a reject like Albon is certainly not going to drive the team forward.
Alonso's not winning another championship (not with Renault anyway), but he wanted to return and Renault wanted him by the sounds of it. He was still considered a top driver when he left so if he's still 90% of that then he'll be better than most. If Ocon outperforms him then it's a mistake, but I with so little testing and an hour less running each weekend I suppose we might have to wait until 2022 to conclusively say that.