Soldered on SSD's is extremely profitable for laptop manufacturers who use such a method because if the SSD fails, it's more profitable to sell the customer a $200-$500 replacement motherboard than a $50 SSD. Even if users have Apple care, having to pay for a new motherboard via Apple care is still more expensive than buying a standalone replacement SSD.
I've been repairing Apple laptops for over 9 years now and contrary to what people believe, the on board SSD does go wrong and it is heartbreaking to tell a customer that they must pay for a replacement motherboard at high expense rather than the much cheaper option which would have been a replacement SSD but Apple does not want that, they want money, they want profit so better to have customers pay for expensive replacement motherboards rather than a standalone SSD.
As a repair technician in my opinion it is criminal what these computer manufacturers are doing with soldered on SSD's. They will try and tell you it's all about efficiency in that it is more efficient to have the SSD soldered onto the board because it would allow for faster transmission of data between the SSD and the CPU/PCH. It is utterly rubbish because the gains from having soldered on SSD to having a SSD via a M2 connector on the board are miniscule.
It is ever so easy to see how the repair industry has changed with regards to customer repairs. A customer brings in their laptop saying it wont boot into the operating system anymore. A check of the laptop shows the SSD has gone faulty. A ring to the customer telling them that their SSD has gone faulty and it will cost $30 for a new one (labor time not being included at the moment). The customer is delighted at the cost and says to go ahead. Now compare that to this different version where the SSD is soldered onto the motherboard, a check of the laptop shows the SSD has gone faulty, a ring to the customer telling them that the SSD has gone faulty but because it is soldered on a new replacement motherboard is required which will cost $300. What cost do you think the customer is going to be happy with?
Apple, HP, Dell, Acer, Microsoft and many more are ripping customers off with soldered on SSD's because the customer has no choice but to buy a replacement motherboard than just a SSD. The motherboard for all intense purposes works, it powers on, displays, keyboard works, touchpad works, audio works, wifi, webcam, battery charging, everything works EXCEPT the soldered on SSD and because of that the complete motherboard needs replacing which will end up as e-waste. It's a disgrace when all it would have taken to get the machine back up and running would have been to replace the SSD, which could have been done before but not now.