The difference between cryonics and, for example, global warming denialism is that the former makes a claim like “it is probably a good thing to do X”, while the latter makes a claim like “X is/is not true”. These are completely different things!
Perhaps it is better to compare it to the anti vaccine movement. They do make a claim of the form “X is good/bad for you”. Now the difference becomes about evidence. For cryonics there is little evidence either way: it has never worked, and it has never not-worked. In such a case there is little we can do beyond trying to use reason alone (always a dangerous thing) and waiting for more experiments.
On the other hand, there is a lot of evidence that vaccines work just fine. Denying that evidence is wrong.
This assumes that the question is asked only once, but then, to which of the 20 copies will it be asked?
If all 20 copies get asked the same question (or equivalently if a single copy chosen at random is) then the utility is (50% 18⁄20 ((18 +$1) + (2 -$3))) + (50% 2⁄20 ((18 -$3) + (2 +$1))) = 2.8$ = 50% * 5.6$.
Consider the similar thought experiments:
I flip a fair coin to determine whether to switch to my headdy coin or my tailly coin, which have a 90% and 10% probability of heads respectively.
Now I flip this biased coin. If it comes up heads then I paint the room green, if it comes up tails I paint it red.
You then find yourself in a green room.
Then I flip the biased coin again, and repaint the room.
Before this second flip, I offer you the bet of +1$ if the room stays green and −3$ if it becomes red.
The prior expected utility before the experiment is:
Given that you find yourself in a green room after the first flip, you can determine the probability that the headdy coin is used:
Which gives a posterior utility: