I think that the transponsons are equivalent to the inelegant excess codes in modern computer programs.
Back when the cutting edge of programming was compiled on IBM 386 computers, programming code needed to be streamlined and efficient. In our decandent modern day of excess computing power, such careful code editing is unnecessary. Now, codes are pasted on top of each other without regard because the excess computing power overcomes programming obtuseness.
Similarly, when ATP began growing on mitochondrial trees, the biological programming code no longer needed to be elegant. The cells could afford to paste DNA on top of each other without dying of ATP exhaustion in replication or synthesis.
It's true that, by and large, the smaller the organism, the less transposons there are, and at tiny scales the organism must be extremely efficient and can't afford 'junk'.