It will never get to that. If the ratio of spendable utxo value to node cost gets too high, the pressure to fork to larger blocks and decentralize the network would be enormous.
The pressure will incentivise a solution. Actually, maybe it would make every node a miner again. If a tx fee is $500k, and running a full node is $200, it might make sense to spend $500k on mining to subsidise your own fees.