It doesn't require that replaceable components are always replaced with non-replaceable ones. The point is that over time, a replaceable component is likely to

It doesn't require that replaceable components are always replaced with non-replaceable ones. The point is that over time, a replaceable component is likely to be swapped out multiple times, until, probably by accident, it is replaced with a component that is no easily replaceable, as which point it becomes stuck (e.g. glue code is written specifically to interface with that component and would be difficult to rewrite for a replacement). In mathematical terms, you can imagine it as a Markov chain describing the component currently slotted in, where non-replaceable components are the absorbing states.