ROTFL. If you're not buying Bitcoins, you're speculating. It's exactly as if you bought thousands of Bitcoins, then sold them for speculation purposes.
So, does that mean I'm also speculating in postage stamps, baseball cards, vintage vases, old wine, and classic cars.... by not buying them? Great :-)
Well.. Take gold for example. Assuming you don't own nor have previously owned gold bullion, you are speculating the price will not hold up and will crash at any time.
Your opportunity cost is a few hundred % over the years.
If you believed it's a sound investment & had the courage to invest in it, you would speculate by buying gold.
Similarly, by not buying relatively cheap BTC at $13 per piece, you are speculating it's overpriced and not worth spending your money on as an investment.
Opportunity cost of not buying BTC at current rates is unknown, but previous people who had the chance to buy at $0.2, $0.5 per coin etc.. Were obviously speculating there would be a bear market & potentially lost tens, hundreds of thousands of dollars.