Oscars are loud as fuck compared to Yasens and are approaching their expiration date
They can have 10 times as many missiles, it doesn't change the fact that time moves on
The Anteys were never the quietest boats even when they entered service and yet that didn't stop them from achieving their tasks. If acoustic detection is all the Antey had to worry about its certainly no issue at all, with modern sonars having trouble beyond 50 miles or so against late cold war vessels, which is by far incomparable to the range at which the Antey with its original weapons can threaten surface targets at, nevermind upgraded UKSK weaponry. If anything, with modern sensor nets and much better stand-off options, the Antey with its deep magazines offers any fleet it serves with a substantial increase in killing power that is just hard to miss. It couldn't replace the Yasen in many missions, but the Anteys are a capable complement; if there are projects underway to have wingman drones that carry missiles for their manned counterparts, why shouldn't the Yasens have one in the Antey? You could offload the majority of the anti-surface tasks to the larger boats while devoting the much more valuable stowage space on the Yasen for purely anti-sub purposes.
Then there is the fact that Russian Navy isn't exactly flush with hulls either that it can afford to offload older but capable boats just because. In an ideal world the Yasen would have replaced the Anteys and but a handful of the newest Schuka-Bs - but this is not that world. There is a Husky being designed to get rid of the legacy Soviet boats but it won't be in steel for many more years while a capability gap exists now, and the upgraded Antey should do its share of bridging that gap while they wait.