In most industrial companies it makes more sense to start as a layer on top, since systems like the CMMS, ERP and document control hold years of data and are tied into many other processes. Replacing them is a long project with a lot of risk, and the AI tool usually does not need to own that data to be useful.
A layer on top can read from the existing systems, do the work that those systems are bad at, like turning documents into usable steps or connecting information across systems, and write results back where they belong. Replacement only makes sense when the incumbent system is clearly the problem and the business is ready for the change.