China's defense spending is not closely tied to annual GDP growth rate, but rather driven by the longer term defense modernization plan. Unless there is an economic hard landing or collapse, which is far from where we're and highly unlikely, I don't expect the annual defense spending growth rate changes all that much; they should hover around 10%.
This growth rate is necessary and sustainable.
As percentage of economy, China only spends about 1.3% GDP on defense, the lowest among any large nations, developed or otherwise. While the West likes to argue that China has hidden spending that may push the real defense spending towards 2% GDP (more a matter of accounting than any "hidden" agenda, similar to if one tallies the "real" US defense spending, it would be approaching $1 trillion). Even so, it's still among the lowest defense spending among major nations. On top of that, China's real GDP size is substantially underestimated (13% - 16%), primarily due to outdated global GDP framework China is currently using (1993 SNA). That system was revised to the
2008 SNA half a decade ago and countries have been making the conversion since, including Australia, Canada, the United States, European Union member states, South Korea, and the United Kingdom. (
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). This means that China's defense spending to GDP ratio is still lower. The Chinese central government has low debt (<25%) and low budget deficit (<3%). So there is no affordability issue both from an overall economy standpoint or the government budget standpoint.
It's also necessary and actually better for the economy if China gradually increases its defense spending over some time period, say, to 2.5% of GDP over 10 years, IMO.
As it stands now, Chinese economy is in a transition period, in need of restructuring (in terms of investment vs. consumption, manufacturing vs. service etc.) and upgrades (more high-tech, more innovative, less labor-intensive). Chinese military is also in the midst of transition, from quantity to quality and from low-tech to increasingly high-tech.
The areas that China needs to invest more and most are in the technology-intensive forces, i.e., air force, navy, space and electronics. They drive more indigenous R&D, which are needed by China and which tend to have more beneficial spin-off effects to the broader economy at this stage of Chinese economic development. It might not be very obvious to a lot of people, but the US defense spending during WWII and the post-war period were actually quite beneficial to the overall economy and technology developments (when it did not overly spend, which was the case during some time periods).
So I don't think there should be too much worries of negative impact on defense spending due to economy slowdown. China just needs to stay on the current steady course - it doesn't need a huge spike in defense spending which more likely cause waste and drag on economy nor does it need to slow down on spending. If there is no major event or crisis (like a Taiwan crisis) to disrupt the current trajectory, in fifteen years' time, China will have a modern and formidable defense force to safeguard its national interest.