If it's not your BTC, then arguably it's none of your business where the money comes from. But given that these are institutional investors, the money likely comes from wealthy individuals looking for better ROI than they might find on the stock markets.
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Just remember that the only one profiting from Grayscale are accredited investors buying shares in the primary market, that is now closed, by the way.
They might even be buying those coins with “in kind” purchases.
Basically: Grayscale is benefiting only those who already are rich, in fiat or bitcoin.