1. Swipe left to invoke the delete button.
2. Press delete.
Since step one is not a simple button (like the trash can in Notes) and it would be unlikely to accidentally invoke it, it is unnecessary to have "Delete text thread?" warning. That would be considered a third step. Any more than two steps is excessive and wastes time.
If you have a Mac computer, read on. If you only use Windows, you may stop reading now (and the why is given at the end of my post).
There is something you can do if you find yourself accidentally deleting threads all the time, but it will (currently) only work for iMessages (not regular SMS).
On your Mac computer, launch the Messages app, log into the same Apple ID account you are logged into on your iPhone. Any and all iMessages you receive on your iPhone will also end up on your Mac computer. If you delete the thread on your iPhone by mistake, the thread will not be deleted from your Mac computer on the Messages app. You have to manually delete the thread from the Messages app even if you delete it on iOS.
When you update to Yosemite and iOS 8, this temporary work-around may change in one of two ways:
Your standard SMS messages, in addition to your iMessages, will be received by the Messages app, so you may end up getting this "backup" functionality for your regular (SMS) texts.
However, Apple may choose to synchronize the deletion of the thread with iOS 8/Yosemite, so deleting it on your iPhone may also delete it on your Messsages app. Someone currently running betas of both may be able to provide an answer for you right now of how this will work, but because these are, after all, just betas, the end result may be different when both iOS 8 and Yosemite are finalized.
However, you're better off to just train yourself not to delete threads accidentally. As has been stated already, you're unlikely to make this same mistake twice, and most people will learn after messing up once, so my method is only recommended if you have an extremely hard time training yourself not to delete threads by accident.
Lastly, if you're running Windows, you're SOL. This workaround only works on OS X (because Messages.app is not available for Windows), and, as I already stated, may no longer work when Yosemite and iOS 8 are released later this year.
I agree wholeheartedly with your assessment, Ledsteplin, and as Ms. Europa has pointed out, thread deletion on iOS is a two-step process, which would save the vast majority of users from making the mistake. It should be quite difficult to execute a deletion of a thread by accident.
I provided an alternative that may come in... handy... for users with special circumstances, such as folks suffering from AHS.