Another way they can become more practical is if I implement client-only mode and the number of network nodes consolidates into a smaller number of professional server farms.
Client-server model is the direct opposite of peer-to-peer model, and when the network of full nodes is turned into "a small number of farms" the system becomes not much different from any other centralized payment system. SPV clients don't validate blocks, they only check if their own transactions belong to a certain block, which means that there are some potential attacks and frauds that might be commited by big server farms of fullnodes+miners against SPV users. But even if this will be solved via some smart cryptography, it would still be a huge security issue - the government would simply take down or regulate those big server farms, turning Bitcoin into Visa 2.0. And SPV clients are absolutely helpless because they are not peers - they can't fork the network if the rules were broken or changed because they don't have the blockchain file, which would be measured in petabytes by that point. Bitcoin today is worth over $18k because people know that it can't be killed - even if China will go after miners, the nodes will fork to a new algo. The only way to kill Bitcoin is to track down every single hard drive with blockchain on it and destroy it.
