This is an interesting and important point. I'll attempt to rephrase your point slightly differently: Everything we observe in the universe has a sequence of l

This is an interesting and important point. I'll attempt to rephrase your point slightly differently: Everything we observe in the universe has a sequence of linear causes stretching backward in time. However we can't be sure from these observations that Universe (or multiverse or something similar) itself has been caused in a similar way. -- I hope I got that right. But is the physical universe (or multiverse or something similar) that we experience a good candidate for the uncaused base reality that just exists? A good reason to think not is that universe is composed of stuff and parts that change relative to each other. If something changes, ie goes from potential to actual, then there is something that is more actual, or more real, from which we should be able to explain the change. Another way to say it is that we may not know what base reality is, but in order for it to be a good candidate for 'the' base reality, it should be completely simple. And the universe as whole, by all appearances, is quite complex.