Why is constant node participation necessary in blockchains?

From Avalanche's whitepaper in the context of Nakamoto consensus protocols:

By construction, they cannot quiesce: their security relies on constant participation by miners, even when there are no decisions to be made.

I understand why blockchain's require active participation from nodes when transactions are being registered (to ensure that the transaction is not malicious), but what is the concern when the system is dormant and no transactions are being created?

If all transactions stopped and all nodes went inactive for a day, what would be the risk during that time / when the system becomes active again? Why can't they quiesce when no decisions are to be made?

If all transactions stopped and all nodes went inactive for a day, what would be the risk during that time / when the system becomes active again? Why can't they quiesce when no decisions are to be made?

There is no risk within the system. However if a blockchain halt for a day then a lot of interconnected world goes boom. You can study this when Solana halted several times over the last few years.

• No payments work
• People cannot move funds in and out in centralised ramps
• On-chain decentralised finance markets go out of sync with other markets
• Price feeds go out of sync
• Collateral liquidations for lending markets may not happen timely; lending protocols go net negative

Generally, one the value promise of decentralised network is 100% uptime due to resiliency from decentralisation. If the network cannot be guaranteed to be 100% functional, this value promise disappears and greatly causes users to doubt the benefits of decentralisation.