They do, as far as I know. Most genealogical DNA testing companies do, and they tell you so. In case you want to upgrade the analysis later.
> doing WGS to do their side business
That would land them in hot water with the EU. Per GDPR, you can't ask for PII for one purpose and use it for something else down the line. 23andMe customers didn't consent to WGS.
But there's another reason I think they wouldn't do that, and that's that WGS is time-consuming and expensive. Some random person's DNA data isn't that valuable. There's a reason payment is part of their business model, and if that's true for cheap microarray tests, how much more isn't it true for terribly expensive WGS tests?
https://services.bgi.com/wgs-sequencing
Making a report is expensive. The WGS maybe cheaper than you think (I don't know how much you were thinking). Then running a quick admixture and some PRS analysis on the data by yourself, with the aid of ChatGPT or some of its friends if you have zero clue, it can be surpringsingly affordable.
BGI is chinese, but there are other services out there doing similar services (e.g. Macrogen in Korea).
Assume you have your WGS files. BGI/Macrogen can already provide you with basic analysis (like the Variant calling), but next you can do is head to https://nf-co.re/ and look for pipelines that might be of your interest: sarek, raredisease, oncoanalyser, hlatyping... and have some fun. Next head to https://snakemake.github.io/snakemake-workflow-catalog/ and look if there is something you can be interested in (grenepipe, dna-seq-gatk-variant-calling, PopGLen...). With some help from a decent LLM you should be able to run your "admixture" that some people loves (the "I'm 3% asian, 24% african, 10% nordic..."). I don't know anyone that does à la carta analysis, we do all of them ourselves, but sure there are some.
Note that you enter a difficult world, and there is no absolutes there: is fun to play, the same way that is fun to track your own sugar levels with an app. But you shouldn't be diagnosing yourself or taking radical actions based on the results. You are not debugging yourself. Soon enough you'll be wondering about your RNA-seq under X or Y conditions.
YSEQ will do full genome NGS if you ask for it but it's expensive, can take six months or more, and the owners have apparently been known to cancel your order and refund you if you complain about the wait times.
We do Whole Genome Sequencing, and sometimes we outsource the sequencing. We always get the excess of DNA back, and it is stored in our own freezers. Even in this scenario we can't be 100% sure they don't store the DNA or the files for their own purposes, but that's the risk we assume. The DNA we send is only identified by a number.
I can 100% imagine a company such as 23andMe storing DNA for later sequencing, or even doing WGS to do their side business, while sending you back only the genotype. Did you request your excess of DNA back? No, you didn't, because you didn't even know how much you sent or how much is needed for a genotyping. What you did was linking your DNA with your real name and some extra data, so further data augmenting is trivial.