Not really "all of a sudden" because russia will need to replace all its radars, all its AD systems and make missiles adaptaded to those new photonic radars which won't be quick nor cheap.
Most of its radars are already tied into air defence systems that combine different radar types with different frequencies... perhaps you are confusing Russia for the US... when the US introduced multiple rocket launchers in their army they replaced tube artillery. The Soviets added rocket launchers to tube artillery and used both together...
When Soviet tanks got modern laminate and composite armour types they used them together with ERA and other things like SHTORA EW systems, they didn't just rely on one solution or another... they used them all together.
These photonic radars will be the same, they will likely add photonic radar to existing systems to add to rather than replace... which will be cheaper and quicker and ultimately more effective for a much wider range of threats and targets.
Stealth works very well against modern targeting radars working in x band. That's why su-57 is stealth.
Many modern radars in Russia don't work in X or Ku band. They are going to make Su-57s which are stealthy, but they are also going to make MiG-35s and Su-35s which are not, and they are going to make more stealthy flying wing PAK DA, but they are also going to make Tu-160M2s which are not.
Sounds like they don't think stealth alone is the answer.
And contrary to what most russian fanboys here are saying, western countries are not stuck with AESA and stealth.
Of course not, if Russia was going for an all stealth and super expensive highly stealth suite of aircraft then they would have to do pretty much what Russia has done with mixed frequency radar that work together with other types to reduce the effectiveness of stealth, but we don't really see Russia doing that and equally we don't see the west doing that either.
And russia is not 20 years ahead in anything.
Well in terms of hypersonic manouvering missiles they have had the Iskander in service for 20 years, and they seem to be progressing in other areas quite well too.
They are also working on quantum radars while russia is still trying to make correct AESA and its new fighters are designed to be stealth which according to the same fanboys is useless. But yet Russia is doing it.
They were supposed to be working on a ramjet powered AAM based on the R-77 but cancelled that... and it can't be because they are shit at ramjet powered missiles... I mean the SA-6 SAM entered service half a century ago, but if it is not that they couldn't do it, then it is more likely that they realised there was an alternative that offered better performance returns... ie a scramjet powered AAM... very similar design issues but rather higher speed performance... why wouldn't you?
The thing with AESA radars is their extreme cost to start out with... the solution is to wait until you have the best possible design out of the right materials and then make radars for ships because those arrays are huge and so mass production is needed and it is mass production that forces down the cost and improves the design and makes them cheaper and more effective. After a while they become cheap and you have learned some new tricks regarding their search algorythms and use makes them even more effective... but the critical thing is cost... PESAs have most of the useful features at a fraction of the cost, and that wont change until you have made a few. Ground based radar arrays and ship based ones means production in the tens or hundreds of millions, which is a great way to reduce to price and increase the performance...
Pretty soon you start integrating these radar antennas into all sorts of things... including HMG mounts and auto grenade launcher mounts and of course drones for armoured vehicles with MMW radar and IIR sensors... which might never be replaced by photonic radars because they do a good enough job not to need replacing.
Is this AESA antenna in the wing, or on it? I am not aware of such AESA on the Su-35 - it might be under development, but it isn't in service yet.
Of course, stealth detecting radar, already shown at MAKS in 2009:
Always been a Russian priority to talk about their most secret operational systems, especially things that give them a serious edge...
So difference between DAS and Himalaya is that the former is an IIR sensor (i.e. a small FLIR) while the other is a dual UV/IR one that operate as an IRST (i.e. gives out tracking data, not images).
But the IR system on the Su-35 and Su-57 also have thermal imaging channels as well as imaging TV systems. They have 360 degree threat warning UV/IR sensors like PIRs in a security system, but their IRSTs are imaging systems and can be augmented with pod mounted systems for lower hemisphere imaging and targeting.
The distributed system is early warning and self defence and DIRCMS related systems, while the nose mounted IRSTs are for targeting ground and air targets.