Can't they? I think they can. The Su-57 is following a typical Russian development cycle. In fact it has had a less protracted development than other late Soviet aircraft. I think the main issues are that i) current production is kinda artisanal. It needs more modern production methods to become affordable ii) the Izdeliye 30 engine is not in production yet. This means mass production will be delayed until those issues are solved. Unlike the USA which made, what, 200 F-35s which are only useful for trainer purposes and cannot be easily upgraded to combat status? Russia does not have money to waste like that. In the meantime they will just continue to churn out Su-30M, Su-35, and Su-34 aircraft which are tried and tested.
Some issues can only be solved with time and throwing more money at the problem won't make them vanish much quicker.
Also imagine one thing. Compute the amount of engines Russia has exported to China so far. How many J-10s alone? Almost 400. That would be enough to manufacture 200 Su-27 derived aircraft. Imagine the Chinese import supply dries up completely and that engine engine production just became available for domestic consumption. Do you use it or continue idling the production units until the next engine generation comes online?
With regards to the stealth bomber I think they put it into the backburner but it was not totally canned. The Russians are suffering from a similar syndrome which happened in Soviet times where they had multiple overlapping defense projects. One example is the supposed to happen Tu-95 engine upgrade. The stealth bomber program was delayed IMHO because of similar issues with engines and maybe yet unsolved issues with the flight control system avionics due to the new aircraft's novel geometry. Say imagine they did the math and figured out that with an NK-32 derived engine the stealth bomber would have less range than the Tu-95. Well the Russians have close to no air refueling capacity and forward airbases. Then imagine it turns out that the design has similar range to the Tu-160M. Perhaps with one Tu-160M refuel operation. Would you continue to pursue the stealth bomber project at a quick pace? When you know that the bomber is mainly of use against 2nd tier nations and that bombing a 1st tier nation will result in a nuclear conflict? Would you insist on production, knowing that you will likely still not have the avionics issues fixed on time, or wait until more advanced turbofan engine cores like the PD-35 become available before you make the bomber thus mitigating the range issues?
I expect Russian exports to rise over the next three years, boosting their GDP, and to also see an increase in their defense spending corresponding to that GDP increase after that period say in 2020-2021. The Russian government is currently focused on strengthening their economic tissue at the cost of defense spending because they consider themselves to be adequately protected with their new early defense network, ballistic missiles, SAM systems and are focused inward. Lately Russia has been doing infrastructure investments which had been delayed for decades. They are also modernizing their industry and I mean it for real. They have enough modernized aircraft to fight a regional conflict. When you look at the amount of military resources they have and what they have devoted in Syria I have to congratulate the Russians for managing to make such significant changes in the course of the war with such minimal resources. Much of the aircraft military industrial complex has been switched to civilian aircraft production to do import substitution. The Russians are hunkering down in case the sanctions war intensifies.
Also I expect to see China more involved in international conflicts over the next decade so perhaps Russia will not be alone in trying to curb USA empire building abroad.
They need to test their military otherwise they cannot make the most of their improved equipment. Already they participate in UN missions and went into that large scale exercise with the Russians. This is just a hint of things to come I think. For example once the USA leaves Afghanistan it is quite possible the Chinese will move in.
Russia is already at $28K on a per capita GDP basis and is still a petrostate even after decades of trying to get away from this.
When you combine that with the low level of R&D spending (1.1% of GDP), an atrocious business environment, and a declining population - Russia is looking at growth of possibly 1.5% per year.
And when you consider how Russia is already spending a really high 4% of GDP on the military, I think it's likely that we will see a sustained decrease in military Russian spending in the next 5 years.
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You also say that a stealth bomber is useless to Russia because it will only be used against 2nd tier nations, because a war against 1st tier nations will end up in a nuclear conflict.
Remember the B-2 was built during the cold war precisely to attack the USSR. And that the B-21 (USA) and H-20 (China) are currently being built precisely for use against other 1st tier (nuclear armed) nations.
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Personally, I still think China will avoid international military conflicts in the next decade. The benefits of getting directly involved rarely justify the costs, particularly when China's strategy is to use its economic heft to curb US empire building and US influence.
Remember that in 2018/2019, China was expected to become the world's largest consumer retail market and also the world's largest R&D spender (as per the NSF report to Congress).
China's GDP in PPP terms in 2018 is also 25% larger. And going forward, these metrics are growing a lot faster than in the USA.
So in the long run, China has the domestic market to support the development of world-class companies in every industry.
China is already the centre of the Asian/Global economy and supply chain, and that won't change even with Trump's trade war.