The problem is that we have texts pointing in both directions.
This link gives a good review:
http://www.patheos.com/blogs/theologyintheraw/2015/02/biblical-support-for-annihilation/
It notes 36 texts that use destruction, death, end, or disintegration. (Actually that’s only part of the list. There are more.) I note that this list mixes indications of conditional immortality with annihilation. They aren’t exactly the same, but I think both would read most of the texts the same way.
However there are also texts that talk about eternal punishment, though sometimes (like the eternal worm) turn out not to look the same when you see the OT background (in that case Is 66). Most of the OT passages tend to be talking about God’s enemies being killed.
And then there’s references to eternal destruction, which is mixed, e.g. 2 Thes 1:9. It could support either the idea that eternal punishment is called destruction or the idea that destruction can be referred to as eternal. The latter seems more plausible, since the former requires reading something into the text that isn’t there explicitly; of course that wouldn’t be the only case that’s necessary.
What do you do in a situation like this? Most people pick one, and find a non-literal way to deal with the others.
Surely we can understand destruction as a way of referring to eternal torment.
In the other direction, eternal is often used of things that don’t last forever. To give a NT example, in Jude 1:7, angels are kept in eternal chains until the last judgment, and Sodom and Gomorrah are examples of eternal fire. Or Is 34:10, where the smoke of Edom’s punishment goes up forever.
So there are possible non-literal readings in both directions. That means that people whose ideas are based upon Scripture have to make a judgement call, which equally good Christians have made in both ways. Admittedly it’s only pretty recently that there have been a substantial number of conservative Christians opting for eternal destruction, but there now are.
I don’t feel strongly about choosing, though I think “destruction” looks a bit more plausible. But what I do feel strongly about is calling one or the other viewpoints heretical, at least if you’re committed to sola scriptura. I’m concerned that in many cases, people are simply reading ideas that they grew up with into Scripture without realizing it. It’s one thing to agree that one has to choose which texts to read literally and which not, and make a choice. It’s another thing entirely to claim that your reading takes them all literally, and that the other approach is ignoring Scripture.
Alternatively they can say that maybe there’s a reason that Scripture uses so many metaphors. Maybe something is going on that we aren’t in a position to understand, and thus that any literal reading will be wrong. The one thing we do know, of course, is that we will be held accountable, and that if we’re found wanting, the results will be a disaster. Perhaps that’s enough.