I will refine the question: do you think bitcoin scales up to it's widest potential user base (everyone) using only block size increases, and remain peer to peer? We're all aware that's not possible, no?
Yes, I think bitcoin can scale to an enormous user base and remain decentralized using only blocksize increases. No, I do not think typical users will want to run full nodes in this case. Yes, I think users will still have access to the blockchain.
This might be contentious, but I don't think we need a lot of nodes for bitcoin to remain decentralized. Even if there were only 10 miners in the world, as long as:
- the miners are in different legal jurisdictions and run by people of different political views, and
- the system remains
open such that if I feel
really really strongly about it, that I can run a node and mine too
then I think Bitcoin will remain decentralized and be vastly better than the present monetary system.
(For the record, I do not want nor expect there only to be 10 miners running full nodes in the future. I suspect there will be much more.)