I guess you are right for the current state of mainnet.
However concepts like sharding, swarmnodes, horizontal scaling and hardware support in the form of JINN (and/or Q?????) will eventually lead to hopefully a highly scalable solution. All these features together will enable some form of collective intelligence where full-nodes are cooperating (without needing to know everything themselves).
A bit vague but these are just my thoughts.
Swarmnodes/sharding: From IOTA's roadmap
Another approach planned to enable the IOTA client running in these very resource restrained environments is to shard the core logic and database amongst different devices that then collectively run it. Similarly to swarm intelligence, this enables a cluster of devices to efficiently make transactions without being a full node, but having reduced trust requirements from SPV and light clients.
JINN: From an old webpage from Jinn Labs (founded by CfB and David)
How powerful will a Jinn processor be?
Jinn should not be compared to processors that you buy for your computer today. Those are based on vertical scaling, meaning that the processors get more and more powerful. Jinn on the other hand utilizes horizontal scaling. I.E the increase in computational power comes from a network of Jinns (distributed computing).
But how powerful will one singular Jinn processor be?
This is unknown and also not important. Only performance/electricity or performance/density matters.
How is Jinn's horizontal scaling different from what is already being employed to mitigate the exhaustion of Moore's Law (I.E. just adding more processors) ?
Answer: Architecture is being developed in such a manner that it avoids limits set by Amdahl's law (http://en.wikipedia.org/wiki/Amdahl's_law). Architecture of x86-64 can't afford this due to marketing issues.
Q: Nothing formal is known about Q. The following article speculates about what Q might be. For sure it will be a game-changer.
“Q” brings distributed computing to the IOTA protocol with JINN processors (or emulators). It scales horizontally, benefiting from the global computational power of all other JINN devices (or emulators).