No currency can provide an ultimate guarantee that it will keep its value forever.
The security and value of national currencies depend on sensible decisions made by the leaders of central banks and the politicians running the country.
Similar to gold, there is no company or central bank issuing Bitcoin or regulating its exchange rate. The monetary policy of Bitcoin is instead, regulated by open source software and mathematics. This means that the supply of Bitcoin can be accurately predicted 10, 50 and even 100 years ahead.
Bitcoin is still young and the role it will play in the future of the world economy is yet to be discovered.
The number of merchants accepting Bitcoin payments can both increase and decrease significantly, and there are currently no 'legal tender' laws requiring merchants to accept Bitcoin as payment. Accepting Bitcoin as a method of payment is completely voluntary.
In the same manner, the Bitcoin exchange rate compared to euros or dollars may also increase or decrease significantly as the price of Bitcoin is determined solely by supply and demand on exchanges - just like how the price of gold increases or decreases based solely on supply and demand.