They have to make a choice on what chain to follow just all other actors on the network. Doing nothing is a valid choice of course, but it might not bring you a desirable outcome.
If there is consensus, then all these nodes add up and have a very big economic vote!
Many people seem to forget that Bitcoin is not pure software. It is a bit more!
edit: typo
The only way you can influence which chain is the longest, without mining, is to pick a preferred chains and sell your coins on the other chain. That is your 'economic vote'.
If enough people do that, one chain will be way more profitable than the other and miners will switch over.