Which states do you consider at risk of leaving the EU for that?

The Visegard states like to bark, but when the alternative becomes being a russian puppet again, they will come back crying and stop trying to play both the EU and Russia.

The Netherlands would commit economic suicide on so many levels when their harbors would no longer access the EU and they couldnt export their own product there either. Also the drug cartels wont be happy at all.

Without the EU Putin would try to reignite the Balcan into war, so his Serbian puppets can scoop up some territory for him.

The western European countries know that their economy would go down in epic proportions, if any of them left the EU.

The danger is that you avoid doing what's necessary, and end up with a status quo that's worse than the change you wanted to avoid. Currently deadlock, infighting, and division mean the EU is increasingly beholden to foreign powers. I mean, sure I'd prefer it if 'my' party had power in the EU. But I'd still prefer the 'other' EU party over China.

Also, judging by your comment, I assume you're also from a country with a (defacto) two party system. But the EU is basically like a multi-party system. So the 'other' EU party, won't be the polar opposite of 'my' party. It'll be a coalition of parties, who have reached their own internal compromise, which means extremes are avoided. Multi-party systems are also usually less dramatic in their swings. Eg. the Netherlands where if Wilders ends up leading a government, his more extreme positions will be tempered by his government coalition partners.

i think the last decade of developments in US politics have provided everyone with an example of where that goes, though. not passing the "sword" policy because you fear your opponents using it doesn't actually matter; your opponents, when they come into power, may just immediately enact it themselves. and if they can't, there's a good chance they'll first enact a policy that broadswords aren't swords, technically, probably, maybe, totally, according to this one precedent from the year 1835, and then enact a broadsword policy.

bad-faith actors, authoritarians, fascists, etc., are more than happy to watch everyone else pull their punches based on some assumption they'll do the same. they won't.

that being said, i can't imagine the veto rule ever accomplishing anything good on anyone's side, really. it favors obstructionism by its very nature, which is inherently anti-democratic.

The problem in this case is that a single veto stops everything. To get anything throu the EU it has to be a compromise. That is just the nature of a multi state system, with even more parties. So usually those laws are not that insane, as the commone ground is generally speaking in the political center. The veto rule gives extremist minorities a lot more power, as 26 of the members can agree, but one can stop it. So if you are not a political radical in the EU political landscape, this rule is good for you.

However members bailing is possible, but it is balanced out with greater unity allowing for economy of scale in a lot of fields. So loosing a few members might very well be worth it for the EU, if it turns the EU into a proper country.