[quote=@Balance]
I just thought of an interesting process of Existence. Assuming that, if a dimension gets advanced enough, it is possible to discover true dimensional travel, then over the many long aeons, surely there must have been many dimensions to discover Existence, certainly more than how many exist now. So I considered this for a while, wondering about reasons, and then I realised: heat death. The imminent destruction of a universe would surely cause all of its residents to leave that universe or die out altogether, if they haven't already moved and blended with the rest of the population of aware Existence. So the number of aware universes would be continually kept down, and of course Tier 5 would keep some universes from discovering dimensional travel, as well.
[/quote]

Not all universes would have heat death due to different physics, but similar processes, or even simply end of time and new cycle. 
Remember that when Tier Five was in control, portals put huge stress on travelers their first time. 
Most likely, the relatively few survivors of their first portals would be ostracized if they tried to teach others and they got killed/went insane when they were taught.
This would likely lead to hermit esque lives, leading eventually to death by discovery of them in a dimension they have an alternate that's the same as them and the others around freaking out, old age, dangers they weren't prepared for because they didn't exist in their universe, etc. 
Or would lead to them finding and staying at a dimension where other travelers were, adding to the culture of that universe of dimension travelers.  

Logically, at least. Not necessarily canon.