Yeah, I would guess there is relatively little overlap between the kinds of people willing and able to pay €650k for a passport, and those who have productive skills the EU is currently in need of. The EU needs more middle-class skilled labor, basically, ranging from skilled blue-collar workers to educated professionals. It doesn't really need Dubai sheikhs or Russian mobsters looking for a passport of convenience.

Would it really be a problem if all of Russia just joined the EU?

Russian mobsters have already bought their way in, and for a lot more money than 650K.

If you have enough money you can buy your way into any country.