I understand what you are trying to achieve. Let me more fully explain the problem, as it is not frustration with having to periodically log-in that bothers me, but the loss of posts. When I submit a reply after my log-in has expired, it takes me to the log-in page to sign in. That's fine. But when that's done, it takes me back to the page to compose the post, and the post that I had composed is deleted. I have to then start all over and recompose the post and submit it again, hoping that the system won't again take me to the log-in page and lose the post. It is the loss of the post that reallly irritates me.
It particularly irritates me because I know it doesn't have to be this way. Other sites handle this better. When you submit a post and a sign-in is required, they take you to their sign-in page, but they keep the post and either automatically complete the post submission after the log-in is done or they take you back to the page to compose the message but the post that was composed is still there—all the user needs to do is hit the submit button again to submit what had been written. While the latter gives users a chance to edit the post one more time before submission, I'd prefer the former approach because it saves having to hit yet another button to get the message posted.
But either way is far preferable to what happens here; there is no good reason to lose the post just to sign-in. It is not difficult, from what I understand, to do one of the two things described in the previous paragraph. Certainly sites less sophisticated, and operated by folks with far fewer resources than the firm that owns this site, manage to do it.