Why old Soviet cruisers are poor modernization candidates

Does upgrading old Soviet-era ships such as Kirov and Slava still make sense for Russia?

No; their hulls and machinery are old enough that rebuilding them is likely less economical than constructing new ships.

The Kirov- and Slava-class ships are reaching an age at which hull and propulsion work approaches a near-total rebuild. Once machinery and structure both demand replacement, the apparent economy of retaining an old cruiser disappears. New construction would be cheaper in principle. Russia's harder problem is that the infrastructure needed to build equivalent large combatants also requires renewed investment. Poor modernization value and limited industrial replacement capacity are separate parts of the same fleet problem.