Well they are going from gas guzzling old airframes to modern efficient jets, albeit a risky type. Apparently the grounding of the entire fleet is Aeroflot playing politics. Specific airframes are affected but Aeroflot, as the largest operator, grounded their entire fleet and are putting in a claim.
When the Avros are replaced their fuel and maintenance expenses will shrink massively....hopefully.
Apparently the Sukhoi can climb AND accelerate at the same time, something the Avro can't! Welcome to the 20th century Cityjet.....