Maybe 64 bit CPUs can get more resource efficient than older 32 bit models. But that's a misdirected use of resources if you use such technolgy to make 32 bit CPUs obsolete. With the same technology 32 bit will always use less resource-demanding than 64 bit. And the claim was that 32 bit has so many use cases (basically all phones and many PCs) that economics of scale are possible.

Of course the nonsense model that you don't pay for Internet services directly, but use Javascript and a lot of other bloat software to run advertisements on your phone to pay via a massive detour is not an energy-efficient approach.

https://en.wikipedia.org/wiki/Embeddable_Linux_Kernel_Subset

I am not familiar with it to say whether it really wasn't that useful for inherent reasons or whether vendors in that space were just more lucky to defend their proprietary solutions than the (mainline) Linux competitors.