The Saturn V doesn't really matter anymore - what matters is that we know it can be done (the Falcon 9 Heavy would be impossible without the simple knowledge that the Saturn V existed), and if need be, it will be done.
The second issue is that no one needs that powerful a rocket at the moment - though I believe if it did exist, everyone would suddenly find use for it (like it happened with PC's and smartphones, for example).
> no one needs that powerful a rocket at the moment - though I believe if it did exist, everyone would suddenly find use for it
This is an important and very underappreciated point in life in general. Asking "will I ever need X?" is not a good idea, because it usually leads to the answer "no", whereas if you actually had X, you'd quickly invent many uses for it. It works this way for smartphones ("why would I need Internet in my phone? I have one in my laptop"), other tools, it works in programming languages - it's what pg calls the "Blub paradox" in [0].
I find that the features of our tools limit our thinking and abilities both as individuals and civilizations, therefore when possible I always opt for the most flexible/feature-full solution.
(there's of course a flipside to that, visible e.g. in the ongoing process of police militarization in the United States - don't give better tools to people if you don't want them to start using those tools more and more)
Several big rockets already were built - and all struggled, more or less, with lack of payloads. First was Saturn-5 - and N-1 arguably was closed because there wasn't enough interest in it, in terms of payloads, for anybody involved. Then it was Energia - again, may be, should USSR survive, it'd still flew, but it was closed, while Proton and Zenit survived. Last was Space Shuttle - which was particularly expensive lately with not enough missions to justify its annual cost.
So, with big rockets, it's doubtful that "build them, and they will be used" approach works. On the other hand, we really don't need big rockets that much anymore - we can do anything with existing rockets. It would be interesting to see how often Falcon 9 Heavy will be employed.
The second issue is that no one needs that powerful a rocket at the moment - though I believe if it did exist, everyone would suddenly find use for it (like it happened with PC's and smartphones, for example).