Quandl is very good and I use them. There are 2 problems however:
1. Like yahoo, they could decide to abandon the system overnight. A new manager might want to take the company in a different direction. In this case you are screwed. You can't assume they will be around for ever.
2. Their database is not comprehensive. Some metal prices only go back a few years; they don't have crytocurrency data etc.
Quandl is a startup that has taken outside investment. I am sure the shareholders would want the company to keep going. I imagine it is a highly profitable business.
Here is the bitcoin data https://www.quandl.com/search?query=bitcoin
I am sure there is a free database on quandl that has metal prices back more than a few years. Try the CME one for a start.