Of course, I meant not Bronze age collapse, but known plethora of existential risks. But your argument that others will outcompete us is valid - unless the totality of x-risks is a universal Great Filter. 

Universe will choose the simplest way to stop time travel. It doesn't care is it the destruction of a civilization or some mysterious way to prevent changes in the past. Moreover, as civilizations naturally have a tendency to fall and this prevents all time machines, then civilization destruction is easier way to prevent time travel.

If a non-cancel principle is false, then causality should move along a timeline twice. First normally, and second time - when the time line is canceled. The interesting question arises: can the canceling wave reach the normal wave and if yes, when? (the answer must be "yes" because if it never reaches the now moment, the canceling never happens). For example, if cancel wave has finite but high speed, it will reach us just before we were going to start the time machine. 

From anthropic considerations, we can say that we will take this precommitment and will follow it. 

Let's assume that time travel becomes possible when an advance civilization reach a rotating black hole, as it follows from general relativity.

However, non-cancel principle is valid and can't be fulfilled by new timeline creation. (That is, equal to Novikov's principle).

In that case, the only way to prevent timeline collapse is to prevent civilizations to achieve blackholes!

In that case, the universe should be organized in the way which prevents large scale civilizations and space travel. This solves Fermi paradox and really terrifying to us.

However, if we precomit never come close to black holes, we can escape the "curse"!

I found it rather difficult to download it from any of the listed sources, except amazon kindly web where formatting is not good for reading. Can you just upload a pdf somewhere? 

Maybe better say 'zoo' vs 'forest', or 'very well protected area' vs 'partly protected area'.
If there is only a few habited planets inside grabby alien sphere, they will be very valuable and very well protected so no UFOs will be observed. 

If there are millions of them, they are less valuable and thus less protected and therefore can be used for some practical activity, like turism, hunting or mining unobtanium. Obviously, if UFOs are aliens, local alien authorities let them be visible sometimes, so local aliens laws are not very strict. 

Observation selection effects like SIA favors the hypothesis that there millions habitable planets inside any grabby aliens. 

I think that your model is correct and 'anthropically' supported. 

In some sense it favors 'zoo hypothesis". However, there is an important distinction: is it zoo or natural reserve. In general, on Earth zoos are rare but well kept and natural reserves are more abundant, but less controlled. The same anthropic considerations which favor silent rulers, favor natural reserves vs zoos.

This has bad consequences for us: natural reserves are more likely to be visited by unauthorized visitors and poachers. Or if we will be less anthrophomorphisng, they have less value for cosmic rulers as they are more numerous. UFOs observations and their alleged connections with cattle mutilations and abductions are more favoring the idea that Earth is less protected hunting ground than well protected zoo. 

Speaking about "unobservable" part. Aliens which consist of fields or using something like "5-th" dimension to travel will have much less visible footprint but will have much large sphere which they can grab, as they can travel with almost light speed. The same anthropic considerations favor such aliens as they will have larger sphere of influence. Observations of UFOs also imply that they use some non-typical for us way of propulsion, like instant acceleration, manipulating gravity and moving through objects. 

In other words, if aliens are not interesting in building Dyson spheres and can travel with near-light speed without leaving visible traces, we will see much less signs of their activity. Maybe they are more interested in controlling space than in performing a lot of computations. 

The conclusion is unpleasant: we are typical and neglected planet which sometimes is abused by our mostly invisible rulers. But it is the same as life situation of most people on Earth. 

I think that UFOs are really a wildcard in x-risk research. 
1.Even if UFOs don't have any serious substance behind them, the fact that many serious military people and even presidents believed in them, should update our prior about human irrationality and therefore increase our expectation that nuclear risks and AI risks will be mismanaged.

2.If UFOs have an interesting, but not world-model-shattering explanation, e.g. they are a form of ball lightings, this opens a possibility of creating new weapons after their nature will be learned. 

3. If their explanation is world-model-shattering, all our expectations about x-risks are wrong. 
World-model-shattering explanations are not only classical aliens. 

They can be:
-glitches and viruses in the matrix; 

-space-faring animals consisting from exotic fields; 

-clouds of nanobots from already extinct civilizations. 

-malfunctioning berserks-robots

-elaborated lies from cold war era similar to other memetic mind attacks like "red mercury" or number radio stations.

It also happens in personal life.

Critics: "‘Long Reflection’ Is Crazy Bad Idea" https://www.overcomingbias.com/2021/10/long-reflection-is-crazy-bad-idea.html 

Actually, I am going to write someday a short post "time machine as existential risk".

 Technically, any time travel is possible only if timeline is branching, but it is ok in quantum multiverse. However, some changes in the past will be invariants: they will not change the future in the way that causes ground father paradox. Such invariants will be loopholes and have very high measure. UFO could be such invariants and this explains their strangeness: only strange thing are not changing future ti prevent their own existence.