Should this behavior from a child be encouraged or discouraged:
They ask you a question, you answer it, but then they ask
but what if [insert clause here]?
You answer again, but they ask
but what if [insert clause here]?
and so on... in a way that feels like it would never end without you saying "that's it for now" or "but there would always be room for another 'what if'!"?
In my opinion (as a parent, not a psychologist or educator), curiosity is not something that should ever be discouraged... except that everything has limits rooted in practicality. What the child is doing with the "what if" cycle is to explore the topic and all of the edges. What if... really means "tell me more about this", and it's something that seems to be helping with strategic thinking from what I've seen in my children.
That said, as you say it will go on literally indefinitely. What I usually do is try to continue the line of reasoning for a while, but if it does seem like it's in "indefinite" territory, I try to zoom out a bit. Oftentimes part of why the "what if" cycle happens is the child isn't able to quite frame their thought correctly - so they keep readjusting until they get to what they really want. Zooming out is useful because it gives them more "big picture" context and lets them explore the answer themselves.
The other thing I'll often do is redirect them to answering the "what if" themselves. "Great question. What do you think would happen?" This does two things - it stops the what if train for the moment, and it lets the child develop their own reasoning skills further rather than just asking questions. I might ask the question in a bit more detail than just 'what do you think', depending on the age the child is and the information the question requires; but often it can be as simple as that. Then when the child gives an answer, if it's incorrect or missing information you can add some additional clarifying information as part of the followup.
To add to Joe's excellent response, but expand more on the redirect:
Depending on the age of the child (~6 and under), I have found sometimes the endless "why?" or "what if?" is perhaps the child practicing conversation. They don't know how to continue the conversation and get stuck in a loop.
What I do sometimes is flip it around and see how they respond. I ask them "why do you think?" or "what do you think would happen if?" I don't do this in mocking or parroting way, but in a sincere interaction of actually soliciting their opinion.
I have often been pleasantly surprised with a response of smiles and giggles and a monolog of their theory of "why" or "what if." If I get a "I don't know" then I sometimes help them come up with some and talk about them.
I have endless patience for sincere attempts to learn.
Endlessly saying "Why?" without listening to the response is a tactic to remain the centre of attention. This is narcissistic and selfish and not something I encourage.
Ask the child to tell you what you just said. If the child can answer, your time is not being wasted. Teach!
For any reasonably-imaginative child, there will never be an end to the "what if" scenarios!
Sometimes it's just a game, like "who would win in a fight between Chuck Norris and a Charizard?" (Chuck Norris, clearly, because he has Great Balls.) In this case you can make your answers as ridiculous as you like. Eventually they'll get tired of the joke and move on.
Sometimes it's genuinely about finding out about the world, and assuming that (as an adult) you'll know the answers. If you do know, then great. If you don't, it's never too early to tell a child that you don't have all the answers, in which case you have an opportunity to show what basic research looks like, and hit Wikipedia or Google. That has the double benefit of not just finding the information, but also putting a delay on the next "but what if..." which has a good chance of them losing focus and going onto something else.
There is a negative version though. My niece is currently having some issues with anxiety, and one of the ways that plays out is catastrophizing. "What if (insert worst-case scenario)?", in other words. Parent provides reasons why that can't happen, or solutions against it. "But what if (insert another worst-case scenario preventing that working)?" Rinse and repeat until the scenarios get more implausible. We don't think she is anxious because she actually believes the worst-case scenarios she's thinking up; we think the anxiety is there generally, and these scenarios are her trying to justify that anxiety. At some point her parents have to step in and say "enough", because talking yourself down a rabbit-hole of negative thoughts is not a good thing. Having been clinically depressed myself, I'm personally aware of how negative ideation can affect you, so stopping that spiral is good.
There's a further possibility too, which is that the child has a genuine and well-founded concern, and one they aren't mentally prepared to deal with. Most obviously right now, "what if Putin nukes us?" In this case it may be necessary to flat-out lie to them, because they don't have the mental resilience to handle the truth that there are some "what-ifs" which we just can't do anything about.
There is a place and a time for things. It sounds a bit in your question like you find these questions fine, but get exhausted after a while. If that is the case feel free to delay the what if activity by explaining in what settings you two can play that game. Like saying: "That's and interesting question, but right now I am tired/have to do something else. We will come back to this question at some other time x." Otherwise the other answers are good. I just felt that this part was missing from them.
I would encourage people ask questions (whether a child or adult). Now what's annoying is you feel you don't have time to think and answer those questions. In that case, just simply reply, "I'll think more about it and will answer your questions later." Then you actually make time to think about their questions and get back to them. Most of the time, they don't care (even your kids.) Over the time, they'll notice. That's magic of consistent behaviors.
I would discourage over encouraging this sort of behavior as this may lead to a sort of learned helplessness. The best approach is to guide and advice the child on how to find solutions to their own problems, and then help them if they stuck in that process itself.
