I think the point is that it is not so easy.
On paper the Exocet should not have been any where near as successful as it was in the Falklands war... both the Sea Dart and the Sea Wolf should have been perfectly capable of shooting them down in numbers, but for various reasons they failed to be 100% effective.
BTW the Moskva... if she wanted to, could carry a Ka-31 as one of her helicopters and therefore have excellent over the horizon detection of aircraft and surface ships via radar.
The S-300 system is a vertical launch system... if you cut that out of the deck you should be able to fit rather a lot of vertical SAM launchers in its place because the original system had a lot of internal space so the missile tubes could be accessed from beneath deck and also clusters of them rotated into position beneath a launch hatch. Those mechanisms and all that free space is redundant with the new missiles so you could pack a lot more launch tubes in the available space.
The main problem with the upgrade of the Slava class ships is that the main armament is angled and external so standard UKSK launchers which are vertical and installed under the deck would be an issue... except on the roe.ru web page it mentions the launchers in question can be angled or vertical and can have any number of launch tubes from 2, 4, 6, or 8 missiles either fixed angled or elevating... it can launch the standard Club missiles including the Calibr land attack missile and the Club subsonic anti ship missile and the combined subsonic supersonic Club anti ship missile too.
Here is the export page for it:
Just replacing the Vulcan missiles with Calibrs or Clubs, and the Rif missiles with modern S-350s would seriously improve performance without much modification at all.
Of course it makes sense to replace the electronics anyway... just for the new systems being so much more superior.... otherwise the single gun turret 30mm weapons could be replaced with Duets and improved radars and EO systems, and of course the short range missile Klintok can be replaced with new model TOR missiles (twice as many in the same space with much better range, ceiling, and accuracy).
Would probably replace the old gun with a 152mm gun turret... getting these things in to service in an upgrade would be much quicker than waiting for new builds, but would not be too expensive and offer serious performance improvements and large boat capabilities that the frigates and corvettes built so far just can't manage... no matter how impressive their armament is.
Could S 300 be guided via proxy? Or Moskva would have to guide them? For ASM just location of Moskva should be send.
The new Sigma battle management system shares data across the navy so any aircraft or satellites or subs nearby as well as any ships will share information about nearby threats or targets.
The helicopter the Moskva carries has a radar (even the Ka-27 has an air and surface search radar) and high flying radar platforms that would be detecting the Moskva could be detected by Moskvas own radars too... with that information shared to other Russian platforms in the area.