Future-proofing a deflationary currency from future counterfeiting
There are a few things one needs to consider:


Bitcoin software is bound to change. It will be adapted to new systems, hardware, and so forth. Expecting a program from now to work 100 years from now is probably not going to happen.
Bitcoin protocol probably will change, but the change will be less drastic. For example, block size might get changed. If a protocol changes, it makes software that uses old protocol not able to properly communicate with the network.
The last part that needs to be addressed, is whether or not the way bitcoins are "stored" will change. At the moment, everything is handled using secp256k1 ECDSA algorithm. In order to spend bitcoins, one needs to sign a transaction with their private key. The exact specific of what such a transaction might look like is likely to change, but the algorithm itself is very unlikely to change unless some serious threat appears for Bitcoin. At the moment, quantum computers are a thing that might or might not brake that algorithm in the future. If that was to happen, the algorithm would be changed.


In order to be able to access one's bitcoins 100 years from now, one would need to get their hands on a piece of software from that time (pretty easy), that works with the protocol at that time (very easy), and the Bitcoin protocol must still accept ECDSA signatures or at least private keys (unknown).

Provided secp256k1 is still secure in the future, your coins will be yours to claim (unless someone adds an unlikely exception of "coins older than X years cannot be spent").