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Just look at NATO stuff like the German Gepard. It was constantly modernized in regard of electronics but got pretty much phased out because NATO AA doctrine switched to missile systems against high value threats like modern era helicopters or jets with some auto cannon functionality on ifvs against Helis.  
Turns out, dedicated radar AA vehicles with auto cannons that have airburst capability are the perfect answer to drones. 
The basic idea of drones is twofold. First they are relatively cheap and stupid, second their price would allow them to overwhelm most missile systems that are more designed to deal with individual high threat weapon carriers or high yield missiles rather than a swarm of low yield targets. 
These auto cannon as systems are turning the cost efficiency back in favor of the defender. 
I bet developers like Rheinmetall with their mantis are salviating at the prospect of this turning longterm with NATO basically building a new iron dome.
Russia makes the bet that Iran can produce drones faster than NATO can produce anti-air missiles. >

Honestly I think they’ll be able to do it cheaper with high volume computer assisted machine gun fire or computer guided anti drone electronic warfare weapons. 
Missiles are expensive but a tracked weapon that either doesn’t need reloading or costs a fraction of a missile is coming. >

cheapest is to kill the source, just need some long range, accurate missiles >

And most effective - I’d go for this option >

Just here to casually remind people that this naval anti-missile system called ‘goalkeeper’ exists.
And while it’d be very expensive to buy even just a few to use in major population centres, Ukraine effectively has a blank cheque from the entire planet to make sure that Russia stays behind its borders. 
But rockets are probably a better option in general. It means militaries who support Ukraine can use this opportunity to donate their ageing stock and replenish with brand-new munitions. >

Yeah Ukraine is basically becoming NATO's dumping ground for their outdated equipment so they can get new state-of-the-art tech themselves.
It's convenient that NATO's outdated equipment is still better than Russia's current equipment though.