I generally don't see why the conclusion is considered to be repugnant not only as a reaction of gut-feelings but also upon reflection, since we simply deal with another case of "dust speck vs torture", an example that illustrates how our limbic system is not adapted in a way that it could scale up emotions linearly and prevent intransitive dispositions.
We can imagine a world in which evolutionary mechanisms brought forth human brains that by some sort of limbic limitation simply cannot imagine the integer "17", whereas all the other numbers from 1 to 20 can be imagined just as we would expect it. In such a world a repugnant conclusion against total utilitarianism could sound somewhat like "Following total utilitarianism you had to prefer a world A where 5 people are being tortured to a world B where "only" 17 people are being tortured. This seems to be absurd." In both cases we deal with intransitive dispositions. In the first case people tend to adjust downward the disutility of a single dust speck so that when we incrementally examine possible trades between dust speck and torture people find that A<B<C<D<E and E<A. The same goes for the second case. People think that 5 people being tortured is less bad than 10 people, 10 people is less bad than 15 people, but 15 is worse than 17 as the last outcome cannot be imagined as vividly as the others.
I don't want to make the case that some moral theory seems to be "true". I don't know what that even could mean. Though I think can descriptively say that, structurally, refusing total utilitarianism because of the repugnant conclusion is equal to refusing total utilitarianism in another world where we are bad at imagining "17" and where we find it absurd that 17 people being tortured could be considered as worse than 5 peope being tortured.
I have tested the theory that scope insensitivity is what makes the RC repugnant, and I have found it wanting. This is because the basic moral principles that produce the RC still produce repugnant conclusions in situations where the population is very small (only two people in the case of killing one person and replacing them with someone else). My reasoning in full is here.
I want to thank Irgy for this idea.
As people generally know, total utilitarianism leads to the repugnant conclusion - the idea that no matter how great a universe X would be, filled without trillions of ultimately happy people having ultimately meaningful lives filled with adventure and joy, there is another universe Y which is better - and that is filled with nothing but dull, boring people whose quasi-empty and repetitive lives are just one tiny iota above being too miserable to endure. But since the second universe is much bigger than the first, it comes out on top. Not only in that if we had Y it would be immoral to move to X (which is perfectly respectable, as doing so might involve killing a lot of people, or at least allowing a lot of people to die). But in that, if we planned for our future world now, we would desperately want to bring Y into existence rather than X - and could run great costs or great risks to do so. And if we were in world X, we must at all costs move to Y, making all current people much more miserable as we do so.
The repugnant conclusion is the main reason I reject total utilitarianism (the other one being that total utilitarianism sees no problem with painlessly killing someone by surprise, as long as you also gave birth to someone else of equal happiness). But the repugnant conclusion can emerge from many other population ethics as well. If adding more people of slightly less happiness than the average is always a bonus ("mere addition"), and if equalising happiness is never a penalty, then you get the repugnant conclusion (caveat: there are some subtleties to do with infinite series).
But repugnant conclusions reached in that way may not be so repugnant, in practice. Let S be a system of population ethics that accepts the repugnant conclusion, due to the argument above. S may indeed conclude that the big world Y is better than the super-human world X. But S need not conclude that Y is the best world we can build, given any fixed and finite amount of resources. Total utilitarianism is indifferent to having a world with half the population and twice the happiness. But S need to be indifferent to that - it may much prefer the twice-happiness world. Instead of the world Y, it may prefer to reallocate resources to instead achieve the world X', which has the same average happiness as X but is slightly larger.
Of course, since it accepts the repugnant conclusion, there will be a barely-worth-living world Y' which it prefers to X'. But then it might prefer reallocating the resources of Y' to the happy world X'', and so on.
This is not an argument for efficiency of resource allocation: even if it's four times as hard to get people twice as happy, S can still want to do so. You can accept the repugnant conclusion and still want to reallocate any fixed amount of resources towards low population and extreme happiness.
It's always best to have some examples, so here is one: an S whose value is the product of average agent happiness times the logarithm of population size.