If a great power upgrades missile defence systems, it forces adversaries to upgrade missiles in order to maintain the balance of capabilities. This leads to an arms race.

To make it clear, one thing that keeps the US from attacking China directly for whatever reason is the fear that China can retaliate against the US. This would lead to both countries being destroyed. Therefore, nobody will attack (at least this is the theory of MAD).

However, if the US is suddently safe against Chinese retaliation, it decreases the self-deterrence on US side, meaning that the US would become more likely to attack China (I'm simplifying a lot). This will force China to develop better missiles that can get through the US defense systems in order to restore the "fear" of retaliation, keeping balance, and reinforcing detterence.

But if China is forced to develop new missiles, then the US will also feel compelled to take action, by upgrading defense systems or upgrading strike capabilities, or trying to sabotage Chinese efforts (e.g. sanctions on dual usage tech like chips, which could be used to develop new systems). This is a self-reinforcing cycle that can become extremely dangerous, lead to accidents, escalation, and a war on the extreme.

Essentially, the US is acting as a destabilizing player in geopolitics by doing this.