I don't see the economics working out. You're going to be competing against the 3090, which has 24GB of memory and costs 3x as much as a 3070 (MSRP), but also 78% more shaders. Just to match the 3090's compute capacity is going to cost you $1000 (or $891 if you amortize over multiple cards). That leaves you $500 for the memory chips, labor, and equipment. According to [1], the memory chips cost $12/GB. You won't be able to reuse the existing memory chips, so that means you'll need to buy 16GB of chips for each of the cards, which works out to $384. At this point there's only $116 left, and I doubt anyone would go for this considering that the 3090 only costs $116 more, but has more memory (24 GB), and has full warranty.
The cost is still going to be high and you risk killing your card. Even if someone offered this kind of service, it would only be taken by a very small number of enthusiasts. So nVidia doesn't care.
If someone is skilled enough to work on surface-mount/BGA components, they likely already know what they are doing. Board level repair vs board level upgrade is a very similar process (to the untrained eye such as mine, at least).
