You haven't given an argument for why "undecidability" would be a problem in this case. Please give a compelling example.
We're not banning regular applications either because they are undecidable.
Also, we could change Turing completeness into something less powerful, for instance programs without loops.
Undecidability means loading your configuration could never complete or could consume an indeterminate amount of resources. Which means it's a security concern as a bad config can DOS your system.
This type of attack exists even without Turing-complete config but just with references: https://en.wikipedia.org/wiki/Billion_laughs_attack
> Also, we could change Turing completeness into something less powerful, for instance programs without loops.
Yes, this is a strategy. For example BPF does not support loops.
Being Turing-incomplete is a valuable feature.