They are already de facto inside. Hence the argument to have a vote at least.

They are already paying the price for not being de jure members. (Currency exchange costs, rate hedging is also costly, and they don't benefit from EU unified foreign policy - the current Trump scare for Greenland could come for them too - hence the argument to join.)

their voting power would be essentially zero and the policy opt-ins very significant

no doubt in my mind I'd vote no in their place

there are members with veto power (for things that require unanimity)

https://michalovadek.github.io/eu-veto-tracker/

which is obviously getting progressively reduced for the things that matter and more things moved to different forms of majority, and for good reasons too because the EU is already dysfunctional enough as is