It could simply be part of the pro version, motivating more people to buy. For me at least it would be a strong feature. Currently I always have to have my mac running, my phone or tablet unlocked and the program running on both machines, just to sync.
Imo there is no need to increase the price, if it increases the sales.
There is already a feature request for a headless server which would enable sync to your own cloud: Headless / Raspberry pi version (for server sync)
Developed once, it would not generate any more costs. The cloud sync service also has hosting and maintainence costs. As a developer myself, I think it should not be that much of an effort to enable the software running headlessly on x86 and arm.
It could be even simpler to provide the ESS a self-hosted option.