But once all of the brush strokes have been made and you're happy, surely it must be possible to merge them into one...
Having lots of brush strokes has got to be bad for performance...
Having lots of separate adjustment pins would indeed be bad for performance.
But making lots of separate dabby brush strokes onto
the same adjustment mask, is no different in principle, than making a similar-looking mask from just a single stroke (by holding down the cursor and moving the brush around).
Each separate History step, along the way, does need to include a separate representation of the changed content of this mask - as a result of that particular stroke - as well as the means to reconstruct all of the other current adjustments at that moment. So those things will take up some added storage space in the Catalog.
But the current operation of Lightroom (what you see and interact with while working on the image) only needs to pay any attention to
how those adjustments currently are. The presence or absence of History steps describing
how it used to be, are beside the point - unless and until, you actually want to get back to how it used to be; or else, to compare how it is now with how it used to be. Those operations, and only those, depend on the relevant History steps having been retained.
Lightroom does not display its image by laboriously replaying all of the History steps. It goes straight to whatever the current settings are.