Let me add a little bit more information about using one chip for RAM and SSD.
Using only one chip not only hammers sequential read write, but random read write as well. During day to day use, programs and system read write small files all the time. Having r/w distributed to two chips double random read write speed, which also helps the performance and overall experience of daily use. Not to mention having two chips also help with wear leveling. Keep in mind, as soon as internal drive dies, your Mac is an expensive piece of paperweight unless being serviced by Apple.
Anyone who claims this issue being blown out of proportion either don’t notice this aspect or don’t care, which then allows Apple to get away with lower cost component while charging more for less. Apple has controlled the high end market mainly because customers collectively let them, not just for their “high quality” products.