The euro has depreciated more (in both real purchasing power and against other currencies) than the Deutsche Mark.
The DM was seen as one of the strongest and most stable post-WWII currencies, while the euro has struggled with crises and inflation.
Bulgaria pegging to the DM in 1997 meant anchoring to a much harder currency than what the euro has become.
Look, it seems you have beef with Bulgaria adopting the euro based on your other comments, but matter of fact is that many people remember the hyperinflation and the Videnov winter — especially older people that were queuing for bread with all of their lifetime savings disappearing overnight. Hence, the board staying almost 30 years. What’s scary for many is that the board in its current form is just a law that can be removed with simple majority, e.g. the current stable state could be made obsolete in a matter of a weekend — with devastating consequences for the economy. Adopting the euro means tighter integration with EU only and it’s mostly irrevocable … sadly for Russia and its proxies over here.