How much value is needed is determined by the society through a free market.
What if they're not a veteran, but just an unfortunate soul with a disability that provides "zero" value?
If you do this you stop getting new veterans. Functionally speaking this is why every society with armies has veteran benefits.
It’s reasonable to assume that the powers that be may find themselves in situations politically precarious if veterans aren’t able to provide for themselves and those they ostensibly fought for. Veterans know where real and metaphorical bodies are buried, they also know that at a nation scale, the internal problems that face first world nations are usually not logistical, but political. If not for fear of disrupting business interests, UBI in the form of food stamps, housing, and Medicare for all is possible. The veterans know this, because they are fed and housed and medically treated at scale during and after service. However, if everyone receives these same benefits also without service obligations, the ability to offer incentives to service is limited.
UBI is a thorny issue due to the complexities of implementing it piecemeal alongside the already-existing status quo. In some ways, a greenfield solution would be easier, but they call those revolutionary changes revolutions rather than evolutions for good reasons.
Some stray links for food for thought:
https://en.wikipedia.org/wiki/Swords_to_ploughshares
The value that any person can create is principally limited by imagination, not the free market. The wonderful thing about a moderately regulated free market is that the imagination of more people can be used to engage the value creation inherent in every person.
