It's not piracy if the company is out of business and nobody bought or took over the old software. Then it becomes public domain.

If the company exists in any way and the software went with it - for example Maxis which is now Electronic Arts - it's technically piracy. You can buy SimCity Classic and stuff off eBay, hell, in sealed boxes even. So in that case you should buy it. But if you can't buy it anywhere within a reasonable distance or online at a reasonable price (at or below the original retail), just acquire it through "special means".