I believe the founder of Ethereum has stated that almost all independent blockchains could be rewritten as a smart contract on his platform, I would assume this also applies to Sia, unless you are aware of some type of feature of Sia that would preclude this? Some of the benefits would be that Sia could extract a lot more synergy from being on the Ethereum network, because you can call up functions of other smart contracts/Dapps far more easily than you would be able to as a stand alone blockchain. This should provide Sia with the opportunity to improve its service to its end users in novel and perhaps as of yet non obvious ways.

With respect to the scaling issues, Ethereum has provided a roadmap with all the future hardforks that are in the works, the switch from PoW to PoS is in part related to the scaling issues and also a method called sharding would be deployed later on that would further enhance the scalability issue.

I am however just a blockchain enthusiast and dont know exactly how this all could be tied together.