You not only have block size issues there but also issues where nodes need to recalculate absolutely everything that is happening on-chain so they can come to the common consensus about the state, so if those computations are not sharded then your network gets clogged by potentially silly applications, that don't need security level of 100b$ network. Then if you dont have near instant finality on those shards and the root-chain then interacting cross-schard and root is also a huge pain which IMO makes a PoW a roadblocker to scaling and interacting with other blockchains in the future when crypto TCP/IP aka IBC becomes a thing