[Malcolm Banks]

Aeshna is a genus of large hawker dragonflies whose larvae are also large. They live in the pond, are voracious predators and tadpoles are definately on the menu. The exuviae are the larval cases (exoskeleton) left behind on vegetation surrounding the pond when the adult dragonflies have emerged to take flight. I cannot imagine anyone not wanting to see the beautifully coloured adults flying around the pond. There may be occasional increases or decreases in their numbers locally, but over years (with all else being equal) these should balance out, the major arbiters being food supply and climate (which are anyway linked), and predator numbers.
At ca 30mm in length, Great Diving Beetles are not difficult to spot, but they tend to prefer larger ponds, so it may be that your pond may be a little small for them. The larvae of the GDB can reach 60mm, and they are also voracious predators.

I think water lice prefer running/moving water but yours could also be waterboatmen of various kinds?

I am not saying that all or any of the above are the cause of your tadpole/froglet crash, but as Dogghound says, predator numbers can significantly affect the number of your taddies that reach froghood. These things have a way of balancing out though, although climate change may adversely/beneficially affect some species more than others. The real sign of damage from whatever cause to your pond ecosystem is in the number and age of frogs you have in and around the pond. Do you have a range of sizes of frogs or are they all fully grown?
M