The current 2% of GDP is a commitment but not a condition. NATO isn't meant to be some sort of club where you have to pay membership fees. It is there for the collective defence of all involved.
Then, given the current state of the world, it SHOULD be, surely?
NATO is an organisation which relies on mutual defence and the willingness of each and every member to step-up and make the ultimate test of that promise, if required: All out war in which your own citizens lay down blood and treasure to defend a fellow nation. (And if taken to the ultimate, Mutually Assured Destruction by default). That's what NATO members sign up to.
2.5% is not an unreasonable national commitment in the post Ukraine world, because its measured on GPD - poorer or smaller countries are making payments proportionate to the size and health of their economy. Otherwise we're in he surreal world in which nation A can choose to spend 0.9% of its GDP on the military and the other 1.6% that its neighbour B is paying, on state subsidised 'nice things'. Nation A can buy votes off of its citizens by having state funded swanky events and infrastructure, feeling cosy and shielded by its more Spartan neighbour who's had to make civic sacrifices up to afford a halfway credible military. I can't see Nation B feeling thats either fair or proportionate to a mutual agreement and the stakes it entails.
In the normal post Cold War world, the nations that spent less of the peace dividend were those who still had significant diplomatic and inclination to military involvement in the wider world - or those who weren't so quick to unlearn the lessons of history, usually because they shared a border with a nation who's spots may not have changed for real
NATO became for many of the more isolationist countries, a convenient cost saving exercise, enabling them to run their defences down on the understanding that the collective force of NATO was just about credible enough to provide the bare essentials. With the frankly lazy assumption of automatic backup of Uncle Sam in the unlikely event anything really bad happened.
Putin has changed all of that. And the incoming POTUS is going to underline it. And tbh, whilst I think blunt hints are an incredibly unwise piece of brinksmanship at this time, I do actually see where that individual is coming from and why it sits well with his voters. Europe,
collectively *has not* been paying its way, militarily. I don't think anyone can deny that, can they?
If NATO *is* for the collective defence of all involved, and all involved are therefore expected to honour that, why are some paying disproportionately far more by population and wealth, than others? When so many nations are under social strain as it is, and when so many others feel a proper existential threat, how can that be sustainable without at least an agreed minimum commitment?