You can buy SSD with the same specs 3.2GB Read and 2.2GB Write for much cheaper. Samsung sells 970 EVO 1 TB m.2 for $300 [1] but you can even buy cheaper on amazon for less than $235. Keep i mind those +$400 was price for upgrade from 500GB to 1TB not the price of 1TB which is much much more.
For example Apple charges upgrading macbook pro 13 from 128GB (silly they even offer pro machine with such small SSD) to 1TB for..... $800! So in other words prices their 1TB SSD disk for ~$1000. It's insane.
Fair enough but Lenovo doesn't solder their SSD. Nothing prevents you from buying the cheapest option and upgrading yourself, e.g. Lenovo thinkpad X1 Extreme has even 2 SDD m.2 slots. You also don't have to max out you storage when buying because you can always upgrade it in the future once you need more storage or when SSD gets even cheaper few years later. On top of that those 2 m.2 slots allows you to put disk in raid 0 or use second one with optane memory once its cheaper and worth it.
[1] https://www.samsung.com/us/computing/memory-storage/solid-st...