S-500 isn't needed for the navy. That's a land vased system for protecting against strategic weapons.
The S-500 will be at the core of the IADS they develop for their carrier groups... it will likely develop into the weapon that shoots down American Zircons when they are ready for active service... they will also defeat any ballistic weapon they might come up with based on IRBM or SLBM designs similar to Chinese anti carrier missiles.
It would also allow a Russian carrier group to be sent to a location mid flight path for western ICBMs and SLBMs and enable them to get shots at warheads as they fly past.
The only "ad-hoc" components of the modernization is the technical specifics of the rebuild. These projects are COMPLEX and its impossible to have fully comprehensive technical documentation in place for fully scope the works required.
They have developed modular weapon systems for all their vessels and also modular sensors and electronic equipment, but it will be a while before they can design and build the custom designed new large ships. In the mean time they have old ships that were in no way intended to carry the new modular weapons and sensors and equipment.
They can either make it a stopgap and therefore essentially fit Gorshkov level stuff, or they can further develop the enlarged arrays and sensors that would be going on their new large ships along with extra weapons launchers over the number fitted to smaller vessels.
Obviously it will take a bit of work to work out how to cram standardised weapons and sensors into a ship that was never designed to take the, so the fit is not going to be snug and perfect, but getting these systems onto the ships and working makes them orders of magnitude more capable than the original vessels with their old gear.
Testing new gear like 152mm artillery guns as well as perhaps Pantsir and improved TOR and perhaps even a few 57mm gun turrets, as well as lots of vertical launch missile systems for TOR and Redut and UKSK means an enormous variety of weapons can be carried, and new much bigger weapon sensors like large radar arrays can be tested on a real ship. New propulsion options can be tested too... the ship with the problematic reactor... take it out and put in a new system that will be used on new ships so it gets testing before the new cruisers are even laid down.
Modifying and testing these old cruisers will give them experience with the modular weapons in larger numbers and bigger sensors and perhaps new reactors with better performance... if it works well then the existing complex system could be replaced and made into a pure nuclear powered ship that is faster and with more available power all the time and of course able to get places much faster with less material needed to support it operationally.
They want to go to nuclear power for the destroyers and new cruisers and carriers so this is a chance to test them out for real...
Right. I would also add that nuclear reactors used to be designed with the idea of lasting around 30 years. With modern upgrades they can last up to 60 years at best.
Actually modern designs could be created that run for 40 years without needing refuelling, which would make them much more affordable and much more desirable too.
The older Kirovs are like 40 years old by now. I doubt they still have much reactor life in them. Even if all it takes is a reactor refurbishment to get them operational when you consider all the changes needed to make the design modern like changing all the cabling, electronics, sensors, weapon systems, etc it makes you wonder if it wouldn't be simpler just to build a new hull.
Even if they started building a brand new cruiser tomorrow it would be 10 years before it would hit the water and that assumes the design is finalised now.
Upgrading an old cruiser will not give you the same quality ship but the Kirovs are big so there is plenty of room... the British often had problems with their new ships because their newer weapons were often bigger and heavier than their old weapons and with more electronics and support systems they rapidly ran out of space. With these Soviet ships the replacement of 1980s electronics with 21st C electronics means replacing building sized main frames with laptop sized single rack computers that are 100 times more powerful than the old building sized systems. The main missiles have gone from 7.5 tons to 2.5 tons and everything is generally smaller and lighter. Improvements in cabling mean a bunch of wires the width of a rubbish bin lid can be replaced by fibre optics narrower than your wrist, yet the communications speed is 10,000 times faster in both directions.
Right now they have some old ships or nothing... with the old ships they are big and available... they wont be as good as a scratch built newbuild cruiser, but they are better than nothing and offer an opportunity to test a range of new technologies and systems without worrying about a from scratch totally new design.
They could develop it in stages... they could use the Lazarev and its wonky reactor as a propulsion testing ship and do some basic standard upgrades. On another ship they could try fitting super large super long range AESA radar arrays for operating S-500 missiles (the 2,500km range system that looks up into space for "targets")... how cool would it be to be sitting in the Atlantic Ocean tracking the ISS as it speeds over...
They could have three different upgrade packages testing different things and when they have a bit of experience under their belts with these upgrades they can decide whether to apply each upgrade to the other two ships... when all the bugs are worked out from all three upgrades they should have some very capable ships and will be much better prepared to design a from scratch brand new cruiser replacement for them...
I think mentioning the S500 in a Russian naval design might not make that much sense because the launchers are supposed to be huge. It is an anti-ICBM weapon like THAAD not an anti-aircraft weapon.
THAAD is barely an Anti IRBM like weapon... S-300V and S-400 are both better systems already... S-500 will be much better... and necessary.
It will be their defence from Zircon type weapons when they become available in the west.