Where? I can only think of three instances of dragons that can be fought in the game none of which are silver. Dragons I am aware of in the game: ::: spoiler Act 1 Spoiler Red Dragon by bridge near the mountain pass. ::: ::: spoiler Act 3 Spoilers Ansur (undead bronze dragon) in the Wyrmway and the red Dragon during the final fight. ::: Do you mean durge? Durge is by default a white dragon*born* which is a species not related to dragons in the forgotten realms.

Fascinating - *witness missed the giant silver dragon behind the group of goblins*.

Ah yes I forgot. Very cheeky our bardic friend.

Fortunately the spell replication is already extremely powerful.

Good thing you can cast Simulacrum first, then use the simulacra to cast wish

If you boss dragon doesn't have time stop what is the point? Wish might be interesting but time stop in combat. Allowing it to recharge it's breath weapon and cast more spells and / or reposition. It's amazing

Feruza Balk's Dorothy was the sort of kid I wished I knew, growing up. It blindsided me completely when, later on, I learned she was the witch I wanted to spoon out my heart for. 🤪

The problem is that you're combining a creature known for being solitary and long-lived with the ability to cast 9th level spells, which are especially overpowered when given prep time. If the dragons are even the slightest bit intelligent, their lairs will be peppered with Glyphs of Warding, tons of True Polymorphed golems, and a series of Simulacrums each copied from the last (thanks to Nystul's Magic Aura letting it count as humanoid). And since the spells don't involve spell slots, those Simulacrums can cast their own 9th level spells. If you somehow do manage to kill the dragon, it will just revive from one of the many Clones in the Demiplanes it set up.

Now you're thinking like a dragon.

Now I'm imagining all dragons just home-aloneing it deep inside a mountain. 

Who could have known that a demigod as old as the world would be really powerful?