You need to account costs of getting caught. Botnets are easily create and maintained fairly anonymously, but renting them out means taking money, and spamming means having customers and sales, all of which increase your chance of getting caught doing something illegal. Doing computational proof of work for electronic cash is very low risk, and at the peak of BitCoin pricing a lot of the hacked servers were being used for BitCoin mining.

Even if you were able to bust a botnet which is mining bitcoins, compared to credit card fraud, bank fraud, this is going to be bottom of your priorities - at least till those setting the priorities own a shed load of BitCoins.

Botnets are often not heterogenous, sure you don't guarantee graphics cards, but most of those I saw were webservers hacked using the same small set of exploits, or same sets of default credentials.

I thought Bitcoins were a proof of work thing, with a limited total number, so the rate of mining changes over time and in response to the price, or has that changed?

When I lasted looked mining on your own electricity bill was madness, but botnets were being used to mine, and had sufficient benefits in being easy to convert that the botnet operators were preferring bitcoin mining over riskier endeavours.

Surely this is just a risk calculation, and depends on many factors.

Currently debt is cheap, so high risk, high return investments make some sense if the expectation is good.

Paying down debt if it is cheap is a silly mistake.

You can still make good returns on the stock market betting that the market is risk averse, we have feeble brains.

Currently the cash you have is probably denominated in a conventional currency which also carries risk, we just tend to assume our own currency is some sort of fix point.

I'll probably have had been buying bitcoins if my wealth was in Roubles, although the Rouble has rallied recently I believe, especially if I was dodgy character with difficulty finding a reliable bank in the West.

Deciding if I should buy BitCoins if I have cash in a reasonably stable currency is another question. Interesting question is when to start buying Roubles, you know eventually Putin will die, or the war will be over.