I get that, but as full nodes disappear and big data stores is the only full nodes left, the model bitcoin uses is no longer the best one imo.
Black and white thinking. The world is full of shades of gray. When the requirements become high enough that is has a real cost many users will not run full nodes but many still will. It doesn't have to be everyone is a full node or the network is reduced to a half dozen nodes. The reality is inbetween those extremes.
At that point an old-style server-client model would do a better job. And with that goes privacy etc.
Well no. SPV clients have methods of protecting privacy called bloom filters.
It could be that having several altcoins would solve this better. The question is how to easily transfer assets between them
Having several altcoins wouldn't reduce the load. If there are x tx on Bitcoin network and Bitcoin is replaced by a number of smaller altcoins, to have the same tx volume would mean the requirements to run all of them is the same. To exchange coins between chains or mine all the chains would require the same amount of resources. If you feel that amount of resources will reduce the network to a handful of nodes then the same thing would happen. If all the mining and exchanging is done by a handful of nodes then you are in the same scenario.
The reality is that won't happen. Higher resource requirements, and many users opting for the cheaper SPV option doesn't mean nobody will run full nodes. IMHO there will always be that critical mass. Enthusiasts, users with large amounts of Bitcoins, merchants, service providers, developers, and just "hardcore" believers will provide enough nodes for that critical mass of full nodes.
In other words right now the estimate on the number of full nodes in the Bitcoin network is probably on the order of tens of thousands of nodes. Lets say 50,000 full nodes and lets imagine the number of users is 500,000. That means 1 in 10 users is running a full node. That ratio will drop if we jump to 1M users we probably won't see a mangnitude increase in number of full nodes, it might still be ~50,000. The ratio of users to full nodes is now 100:1 but we still have 50,000 full nodes in 200 or so countries to provide decentralization. Jump further in the future 10M users, 100M users, etc the ratio between users and full nodes is almost guaranteed to drop but that doesn't mean the number of full nodes will drop from 50,000 to 50.