What will happen when the blockchain doesn't fit in a median harddrive?
The decentralization narrative will go away. Only a few people will have enough storage for the blockchain and you will go back to having to trust someone.
With a pruning client you only need to store a small subset of the blockchain and there are no major changes in regards to trust. Your client still verifies every single transaction, but it removes data from the chain that it won't need anymore in the future (e.g., an old transaction that already has been spent).
With a SPV client your client only stores the chain headers and then only requests the required data from a server. SPV clients need a small amount of resources and can run on mobile phones. At the same time most of the important functionality (transaction signing, transaction verification, ...) happens still locally and the level of trust that you need to put into the servers is quite small.
The blockchain is currently at 160~GB, and I believe it will grow around <60GB a year. [1] In 10 years it may be ~760GB, but the median hard drive shouldn't be too far from this.