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Waston pharmaceuticals, one of the largest generic drugs makers in the US has total sales of $4.6B. Pfizer, one of the biggest drug companies in the world, has an R&D budget of $8.5B.
Even if you doubled the price of all of Waston's drugs through a tax and gave that to Pfizer, you wouldn't even cover half of their R&D expense.
Without a patent system, there would be zero incentive to create new drug. A drug company could spend $100M to get a new drug to market and with the typical 8 years of patent life, charge the exact price (let's say $100/month, with $90 being profit) to recoup their expenses (no long-term profit). It would be a SIMPLISTIC exercise for another company to come in and starting selling the drug for $20/month and make $10 in profit, having the benefit of never coming up with the $100M to get the drug approved.
I agree that there are alternatives to the current patent system, but like democracy, "it may not be perfect, but it's the best system so far".