Halvings do not necessarily reduce difficulty used to protect the network, if we can assume price growth by time.

But it does reduce the relative amount of resources used to keep the network secure.

In the end, pow networks are secured by financial incentives.

What is the relative amount of capital needed to protect blockchain to avoid malicious attack?

We need to realize the fact that any pow network can be attacked by wealthy individual or organisaation.

Reducing mining rewards make this attack cheaper.