Is UBI (Universal Basic Income) the Only Way in a world of AGI?

No.

But, more specifically, the only way to what end?

If the AGI is willing to do all the work to create all the stuff, then UBI could work. But if an AGI is possible, and is actually created, why do we assume that it's going to do exactly what we wish?

And, in a world of AGI? Well, we don't live in such a world, at least not at the moment. At the moment it's as relevant as saying that in a world of vampires, the only way is to carry garlic at all times.

Will we ever live in a world of AGI? Will we live in such a world in our lifetimes? Maybe we should answer those questions before worrying about "what if".

UBI is optimistic because it recognizes that people have value and deserve a chance to live a decent life, thus the requirement to give them an income and let them use it as they like.

The reality would probably be far worse: https://marshallbrain.com/manna1

In the short-term, UBI will work. In the long-term, when generations of people are on UBI and don't want to work, the system will either collapse or government forced-labor will need to be enacted to pay for it.

UBI is not a viable nor sustainable business model. It is a utopia. Regardless of advancements in other areas. Because a) economics b) human nature.

By offsetting the temperature scale you don't make anyone warmer.

Maybe, but if were were to implement a UBI, it brings up so so many questions about how would we pay for it, what about inflation, what keeps anyone in a job, and many more, if you think it's a solution, I'd be interested in hearing how you might deal with these issues, but I think a UBI is reserved for a sci-fi future that's never going to happen.