Funny question 
I'd agree and Follow that a bitcoin might be said to last forever just like 4 + 2 will equals to 6 forever. But the ability to spend or use the bitcoin will be much shorter.Spending bitcoin is done through the medium of DIGITAL CONTRACT of a transaction, by which the value transfer from one address to another address.For performing all these operations, you use your private key (which must be stored in some secretplace). Then the signed transaction must be broadcast onto the Bitcoin network and be mined. As you can see there are many things which must work together in order for you to transfer bitcoin. 
I'd agree and Follow that a bitcoin might be said to last forever just like 4 + 2 will equals to 6 forever. But the ability to spend or use the bitcoin will be much shorter.Spending bitcoin is done through the medium of DIGITAL CONTRACT of a transaction, by which the value transfer from one address to another address.For performing all these operations, you use your private key (which must be stored in some secretplace). Then the signed transaction must be broadcast onto the Bitcoin network and be mined. As you can see there are many things which must work together in order for you to transfer bitcoin.