Depends from the aircraft. In early game Il-18 is a fantastic aircraft, and even in later game TU-204-220 can be a good option as poor man's 757. I even operated TU-154M for a while successfully, though I didn't have much competition on most routes.
Russian steel has its own rules, and amongst them is that their maintenance cost grow prohibitive when they grow older. So it can be wise to ditch them at 15 years old, before the second D-Check. Which is not always a good idea for western planes. Which means you're replacing them more often. And when you're replacing, you're not growing. Another reason to dispatch them earlier is the fuel prices evolution.
It's no true for the last generation of red planes(especially the SSJ), but often true for the previous generations. Of course, some planes are much better than others.
I'm tempted to try the TU204, sooner or later. Statistically, it's a cheaper 757 with shorter legs. As the 757 is vulnerable on longer routes, due to the "too small penalty", it seems a nice alternative anyways. Though later, on most routes, the last generations of 321/739 are nearly as big, and far more fuel-efficient. Three times costlier to buy-lease, though.....