Does there exist a formal definition for (what I would say is) a logically fallacy that would fit to the following structure of statements:
"X has not happened (with potential time constraint Y), so Z will not happen (now or in near future)"
As an example of sentences that I have in mind:
It appears to be an argument from ignorance.
From Wikipedia:
Argument from ignorance (from Latin: argumentum ad ignorantiam), also known as appeal to ignorance (in which ignorance represents "a lack of contrary evidence"), is a fallacy in informal logic. It asserts that a proposition is true because it has not yet been proven false or a proposition is false because it has not yet been proven true. This represents a type of false dichotomy in that it excludes the possibility that there may have been an insufficient investigation to prove that the proposition is either true or false. It also does not allow for the possibility that the answer is unknowable, only knowable in the future, or neither completely true nor completely false.
I do not see a fallacy in the reasoning, although the absence of a fallacy does not mean that the reasoning is correct.
The problem looks to me like an inductive conclusion (a prediction of the future) has been drawn from a database (one failed event) much too small to ever justify it. A larger bank of observations, even including the failed event, might be enough to allow an estimate of the probability that the conclusion is reliable.
A formal fallacy is one which is made independent of the meaning of the claims and is a function of the logic applied to the terms itself. These fallacies are syntactical. For instance, affirming the consequent:
A -> B
B
Therefore A.
The fallacy of which you give examples would be considered informal because the lack of warrant derives from dubious connections about the content of the propositions themselves as well as unstated premises. The art of informal fallacy is a little more difficult since it is semantic in nature, and generally requires eliciting those unstated presumptions. It therefore can be intuitionally clear that reasoning is poor, but a little more difficult to pin down why. The lazy thinker might be quick to dismiss this as non-sequitur, but the retort is simple: that the content of the antecedent and consequent aren't entirely estranged. In both examples, a case can be made that there is an analogy at play, in the first as examples of aerospace engineering, and the latter as cellular pathology. So, the first step in your examples is to ask:
What is the causal relationship between the failure of predictions about flying cars and space tourism?
It seems reasonable to expose the reasoning as such:
P1: Flying cars have been predicted since I was a kid, and they still aren't here.
P2: (Designing and implementing flying cars and space tourism is essentially the same thing by analogy).
C: So we will not see space tourism in our time (since the failure of the former is essentially the failure of the latter).
and
What is the causal relationship between failing to cure the common cold and curing cancer?
as
P1: We can't even cure the common cold.
P2: (Curing the common cold and curing cancer are essentially the same thing by analogy).
C: So cancer will never be cured (since the failure of the former is essentially the failure of the latter).
Note there seems to be two distinct fallacies at work. The first is more obvious. To presume that in each case the prima facie similarity supports the notion that they are essentially the same thing is fallacy by false analogy. That's easy to deal with by showing that the technological processes behind implementing each type of aerospace product are fundamentally different or that the common cold is different than cancer since the former is a specific class of viruses and the latter is an aggregated set of dissimilar cellular malfunctions.
The second fallacy at play here is the notion that because something hasn't happened, it won't. This seems to be a temporal-causal version of absence of proof is proof of absence which is the fallacy of evidence of absence. In this case, the failure to provide an aerospace entity or a cure for a disease is taken as proof that the process itself can't be done.
As stated before, informal fallacies are more difficult to classify, and there may be another good interpretation of the specious reasoning at play. Generally, the point of philosophical discourse is to elicit these unstated propositions that are part of an argument (sometimes cast as enthymemes) and to provide more explanation or more precise definitions.
