Yes, Rhys - you are quite correct. However, last time I was there (a fair while ago) both the plaque and the gate were totally obscured by flood debris. No great problem because, as you point out, entry was easily attainable via the actual stream sink. (Not sure what the current situation is).
Several interesting caves in the Nature Reserve at Gwenlais (close to where I live) do in fact have small signs outside their entrances. These were placed by the CCW and simply advise against exploring the caves without proper equipment, also giving a contact number for further information. There are no gates but there would have been had I not become actively involved in the access negotiations. Although a little intrusive, the signs seemed a reasonable compromise - certainly preferable to several gates! Sadly, not everyone agreed and the sign on the largest entrance (oddly enough, the least noticeable) soon disappeared but the others are still in place. (And no tourists have gone missing yet, or at least none that we are aware of!).
Stuart France recently reminded me that he once erected an informational sign at Agen Allwedd following an incident of damage to the gate but it did not last long, a couple of months at best.
So signs are not necessarily going to solve every potential future problem but I do agree with both Rhys and Shortscotsman that having one there initially is a step in the right direction.