You own it forever, but only on your current computer. When you bought the song, a license which allows the song to play was delivered to your computer (you cannot transfer the license to a different computer).
I doubt that you will still be using your current computer within 10 years. When you transfer that song to another computer, the license does not transfer (and even if you did transfer it, the license would be invalid on that computer). When you go to play the song, iTunes will notice that the song does not have a license, and it will then go to the Apple license server to request a new license for the song on that computer. If you don't have any more computers left on your license allocation (the license server will grant a license for up to 5 computers), a license will not be granted, and the song won't play.
This implies that as soon as Apple pulls the plug on that license server, you will be unable to transfer any of the iTMS store songs to play on another computer, as iTunes will be unable to obtain a license for that different computer.
By the way, this isn't a slam against Apple. The same situation exists with every other online music store that is selling protected DRM content.
And, this will happen. Support for current formats will eventually die and at some point, you'll be unable to transfer these songs to newer or different computers. I've been using Apple products for a long, long time and they do stop supporting old stuff after awhile. For example, my Apple eMate 300 is less than 10 years old... I'm pretty much out of luck these days trying to get a replacement battery from Apple.
But the point is, the iTMS songs that people are buying are unlikely to be useful for a lifetime. And, if you're someone who is buying a lot of content, would you rather spend, say, $1200 for 1200 songs that will be useless in 10 years, or pay $10 a month for access to hundreds of thousands of songs?
Some may argue that the subscription model is a better bargain. And, it's not like you're throwing money away. For that $10 a month, you're getting the entertainment benefit from listening to that music. If you pay $50 to go to a live concert, do you demand a video recording of that concert so that you can enjoy it over and over? No, you pay the $50 for the enjoyment of that experience of the moment.
Some people don't go to concerts and only buy recordings. Some people do both. The point is, why does there have to be only one way? Selling downloads and offering a subscription service do not have to be mutually exclusive.