If I had a request to upgrade an M.2 NVMe drive, this would be part of the price I'd charge for the job. When the new drive was in place, the old drive would go in the enclosure, formatted, and become a very fast external for the client. Though a lot of the time, clients don't care about the old part and just tell me to keep it. Which means the next person who has the need gets it done for a lower cost as the equipment is already paid off.
Seems like the loser here is Optane, which is currently getting business from caching hard drives. As 1TB NVMe drives become affordable for mainstream use, more and more models are going to forego having a 2.5" drive bay. I can see laptops starting to have two or more M.2 slots. Most will stay empty but will be a useful option for those ordering or upgrading for high end use. Something like a big gaming laptop might have four or more slots, with some of them limited to SATA to work within the limited PCIe lane availability until PCIe 4 reduces the needed lanes for each NVMe device.