Democracies are notoriously slow to react to outside threats, but when they finally do, they can be incredibly effective. See: WW2. This doesn't mean it's inevitable, but the thing with democratic countries is that they are also by far the most successful and resilient economies and leading in research thanks to their strong institutions. Meanwhile, strong institutions and research are threatening to autocratic rule, which means they are constantly hampered in countries like Russia and China - while the rampant corruption inherent to autocratic regimes limits economic success after the initial mass mobilization boost. You can see both of them falling behind already as Xi's and Putin's grip on power is intensifying. A typical symptom of this are mandatory ideological classes being rolled out and taking up vital time in the curriculum.

Both countries are also hampered militarily by corruption and nepotism - and loyalty being valued more than actual competence is an inherent disadvantage, since competent military leaders are an inherent threat to any autocratic rule. Russia's embarrassing performance against the much smaller, poorer and weaker Ukraine even long before significant Western help arrived is an obvious symptom of this, but China is even worse in this regard. We haven't seen their performance in an actual full-scale war yet, but previous military encounters - e.g. during UN missions in and around Africa - showed just how incompetent, inexperienced, poorly trained, -led and -equipped the Chinese military is.

Another aspect to democratic rule is that it remains so attractive that even the majority of autocratic regimes at the very least pretend to be democracies. No amount of democratic backsliding in the world changes the fundamental appeal behind the concept or democratic participation, of each individual citizen having a say in matters of state.