Destroyers and I think airships can detect subs, along with other subs. Only SD's can detect SD's. Subs will always have to worry about standard Destroyers. As of 3.17 they are now a dead end unit that never upgrades or goes obsolete. In 3.17 I believe SD's were stripped of their ability to shootdown planes and Destroyers were therefore allowed to be built until the end of the game, they no longer upgrade to SD's. SD's are a weird unit, but great for scouting or (I imagine) blockading. They don't seem to really have much to do with Destroyers anymore actually. Mobile artillery are pretty sweet. Once the enemy develops AA defenses, you want to use your precious air force more carefully and having arty around to bombard defenses to 0% without any risk of retaliation is very useful again. Obviously, as you indicated, there's a trade off in that you're delaying other techs.