My 2 sats.
The question might be why do we need to scale? Is it improving the current functionalities of bitcoin (faster transactions, denser transactions…) or is it about new/different functionalities? 
Maybe rollups are not suited for scaling core bitcoin functionalities. 
I must say I have not yet experienced a situation where I needed a smart contract chain. 
Confidential transactions might be an exemple. 
** From a casual observer of the space **