The value of Bitcoin is comparable to that of gold. Bitcoin cannot be used as legal tender. Even the legal currency of a country is to save the country's economy and does not want to rely on the US dollar for development.
Bitcoin is an emergency safe-haven solution. It cannot be said that Bitcoin is the future of money, it can only be said that Bitcoin will exist for a long time in the future.
The whole '"legal tender" ordeal isn't actually relevant for Bitcoin. Legal tender, for all intents and purposes, just means the government has a stamp of approval on a currency which can be used for transactions. If you're issuing funds to the government or by order of government to another party, you can only use the currency regarded as "legal tender." For anything else, it's up to you and the other party what currency you use.
Of course, you don't actually need the government's permission to conduct commerce in free countries so whether Bitcoin's classified as legal tender or not is entirely irrelevant. Most transactions are private in nature and therefore the currency does not need to be legal tender.
Bitcoin will exist forever. Whether or not people use it the software will be there and be forever accessible. Fiat currencies exist in a transient state because governments are forever temporary. There is no guarantee that even the strongest currencies will, for certain, exist in the future because that would require with absolute certainty that the issuer will continue to operate.