Russia's Su-35 Fighter has equivalents in the current Russian Air Force with planes with the same capability but cheaper. Based on this, keeping the older aircraft around might not be the most practical thing to do.
However, operating costs are tricky. Even the US Air Force (USAF) has this problem with its most expensive yet capable planes like the F-22 or F-35.
A case in point is the Su-27 which comes cheaper for the long-ranged operation, with Su-30s having better strike performance with the current variant.
Too many Flankers around
The jet is one of Russia's best fighters; most of the fleet is mostly older planes. The International Institute for Strategic Studies (IISS) mentioned that 220 legacy Flankers to 70 Su-35, nearly triple its number in 2018, reported the National Interest.
Legacy Flankers have considered the first Su-27s, and its upgraded types, and the 2-seater Su-30s. However, these older planes are getting obsolete or are not close to the Su-35S.
About 50 airframes of the Su-27 are what Russia has now, which are the basic Su-27S which was introduced way back in 1985. Another variant in the list is the Su-27UB or a two-seat version of the jet. Overall, basic Flanker is not good with outdated radar onboard, unlike modern EASA radar today.
Offensive and defensive capability
A Su-27S can only fire semi-active radar homing missiles like the R-27, which needs the pilot to aim the radar the whole time to hit a target or lose contact; it hit nothing. The fact pointing the nose to the target makes it not very convenient, but the Su-35 Fighter, which is newer, fares better.
So, the R-27ER has a more extended range, noted Military Wikia, and midpoint functions to the plane, which is too old by today's technology. But for today's air combat, it would be a sitting duck without self-guiding missiles that active homers.
Though it will be different if a Su-27S will be in its fighting environment, especially the dog fight close and personal, the development of the basic Sukhoi airframe gives it grace in a tight battle that some modern planes lack.
When most planes would tangle with the Su-27S, the helmet-mounted sight and the R-73 is an excellent combo but will not hack it now. Newer technology has negated IR missile tech by introducing the AIM-9X with Joint Helmet Mounted Cueing System (JHMCS), which automates the process.
Close air combat with the JHMCS will give a capability to engage at more angles, which a basic Su-27S cannot do and will end up getting shot down.
Air to ground missions
Another role of the Su-27S is for attacking ground targets with basically dumb bombs or unguided weapons. Because of this, the fighter is categorized as a pure "fighter" in "The Military Balance," whereas other Flankers are designated as multirole.
One of the first Flankers meant as fighter and strike aircraft is the Su-27SM, cited Airforce Technology, with forty-seven types to date. It is an upgrade of the Su-27 that has vastly made it more capable than earlier variants, with better avionics.
The Tie in Su-35 or the modernized Su-27
The Su-35 Fighter is the most modern Russian plane compared to the Su-27SM, cheaper with everything the newer aircraft has. Eventually, it will cost to have all the legacy Flankers replaced, which is a big deal.
