Just to be clear, sandboxing is possible with Guix, with least-authority-wrapper as a built-in option. Regarding the long term vision of capability security, you can read the Spritely (the nonprofit I work for) whitepaper about capabilities and the work we're doing in Guile to make it happen [0]. The paper isn't about Guix, but Guix stands to benefit from the effort. Getting to the point where Guix services are capability secure will take many steps, but one step is bringing capabilities to Shepherd, which we have made progress on through an NLnet grant [1].
[0] https://files.spritely.institute/papers/spritely-core.html