Been lurking here for maybe a year now. Odd what topic will make somebody want to register and post.
I've been working on my own trailer recently, and also work for a company where one of my many responsibilities is the maintenance, transport, and security of a half dozen or so trailers, so I suppose this stuff is kind of on my mind lately.
Those shank style tongue locks are just no good. I know the intent is to stop the casual thief, but that depends on what your definition of casual is. I'm certain I can pop one of those things off with a decent pry bar. As you've handily shown, an angle grinder will do the job in seconds. I got a cordless angle grinder for 50 bucks. Pretty cheap for a universal key
If you're in need of a new lock, I really can't help but suggest a product like the BlockHead by DeadBolt; or if you're really serious and think it's worth the money, something in the Atwood AMPlock series. The nice thing about the DeadBolt stuff is that you can also get couple/latch locks that are keyed the same. Nice to be able to stash one in the vehicle somewhere for those moments when it gets misplaced... although I suppose that doesn't help if only half the key is misplaced inside the tumbler.
Of course, you do all this, then the really enterprising individual can just unbolt the coupler itself.
I really can't recommend wiring a trailer to couple to your vehicle's security system enough, especially when it's storing several thousands of dollars worth of equipment and sitting in the alley or backlot of a bar.