My problems is probably the same as most people no organism we see around us today has survived the 4.5 billion years their genetic sequences arguably has but not them unchanged.
However, they are all descended from ancestor which first arose ~4 billion years ago. Take any living organism alive today - whether it is a bacterium, or a human, or something else, they are all linked by an unbroken chain of living organisms back to the last common ancestor. Yes, we are different then our ancestors, but at no point is there a stage of non-life between any extant organism and the first organisms on earth.
So calling any organism "higher" or "lower" is naive at best - we've all run the gauntlet of evolution for the same length of time, and all been successful at it. The only difference between "higher" and "lower" is the number of cells we happened to end up with.
The oldest organism I know of unchanged is a stromatolite but I guess there may be some simple cell stuff that really does go across the 4.5 Billion years and so really has been successful as you define it.
Actually, the bacterium which form stromatolites evolve rapidly, as do most bacteria. They have stumbled on a good survival strategy, so that aspect of their biology hasn't changed - but the remainder of their biology has evolved quite significantly. Much of what we know of cyanobacterium evolution has been discovered by studying the cyanobacterium in stromatolites. You can actually track their evolution by comparing sequences from different depths within the stromalite.
Certain genetic sequences and approaches may be successful if we are defining existing for 4.5 Billion years but you can't really extend that to the organism.
You're making a fundamental mistake here - assuming the genomic elements are more stable than species. The opposite is true - genomes change far faster than species. Life is not a static thing - it changes with every generation. Ergo, one should expect change over time - stasis = extinction, change = a chance to survive. The plasciticty that evolved early in lifes development is the very key to its success.
Secondly you equate bacteria and more complex multi-cellular organism as the same based on one criteria that we are here today so we are all successful. In itself nothing wrong with that from a survival point of view but it overlooks the battle the more complex organisms underwent to be here. The more complex an organism the more thing there are to be attacked so the more advanced must be the mechanisms to protect it. It is therefore natural to view complex organism as more advanced because they face tougher risk/reward equations.
Sorry, but that is completely backwards. Selection forces on bacterium are far stronger than that observed in multicellular organisms - indeed, the rates of genetic change, speciation & extinction are orders of magnitude higher in single celled organisms; a clear sign of stronger evolutionary impacts measured over the same period of time. To put it into context, your average bacterial species lasts from tens of thousands to a few hundreds of thousands of years; your average metazoan (animal) species lasts from 2-3 million (mammals) to 16-25 million years (formernera).
I guess the way to pose the question back to you both is why do complex organisms exist at all, what is the payoff for being a complex organism and why did it come about?
The first thing to keep in mind is the vast majority of organisms - whether measured as total number of individuals, total number of species, or total biomass - are single-celled organisms (on a scale of 100:1 to trillions:1, depending on your choice of measurement). Meaning that multicellularity, as a survival mechanism, is not overly successful. The question isn't so much why it came about, but rather why it is such a rare evolutionary path.
In terms of why it arose, predation is thought to be the primary driver - unicellular predators like amoebas cannot eat things larger than a few cells in size, so a simple multicellular organism like a sponge, or a pseudo-multicellular organism like choanoflagellates, can avoid a strong selective force simply by "clumping". After that, in-clade competition (i.e. competition between multicellular organisms) appears to have driven much of the rest of the evolutionary processes. Indeed, if you look at most of what has happened since we became multicellular, most of the adaptations have been to compete against other multicellulars.