Bitcoins are stored in the blockchain. As long as at least one person holds a copy of it, all Bitcoins will remain in place no matter how many people are in the network or if new software is still being developed.
As the main client is open-source, even if it was discontinued anyone can pick it up and continue development on their own. Even if that was unavailable, the Bitcoin protocol is not too complicated, so one could create a new client from scratch if needed be.
Lastly, the main client is pretty mature at this stage, so even if no further versions were developed, it could provide enough functionality for a long while.
All in all, as long as you store your own copy of the blockchain and a client, your money is completely safe, at least in regards to the amount of it you have. The value of the stored Bitcoins, however, can always drop or rise regardless of the state of the project, which is another factor to consider.