We have been running a business on exactly the opposite premise for over 20 years, and we're not really seeing that problem with the adoption of Proxmox VE or our other (AGPLv3) FLOSS projects that we provide commercial enterprise support for. Very rarely, a potential customer doing extra due diligence, and not yet used to working with FLOSS much, wonders about exactly this - which is also the main reason I bothered writing anything here, as spreading such FUD further only hurts everyone involved. In those cases, a simple reply that a) this makes no sense (more on that below) and b) an assurance in writing that none of their software running on top of, or interfacing with, any of our APIs has to be relicensed settles it. That assurance is technically useless, as it doesn't grant them anything extra, but it costs us nothing either.
And the reason it makes no sense is that by your logic, any tool that can consume some (e.g. HTTP-based REST) API would have to be open-sourced the moment I get it to fetch data from my AGPLv3 service. No offense meant at all, but that would simply be bizarro world, and there is enough precedent against it. If APIs did not act as clear boundaries, AGPLv3 software would indeed spread to everything it touches or traverses, like an epidemic. If yours or your customers' lawyers claim otherwise, you/they should get ones with an actual understanding of software and IP law.
And the reason it makes no sense is that by your logic, any tool that can consume some (e.g. HTTP-based REST) API would have to be open-sourced the moment I get it to fetch data from my AGPLv3 service. No offense meant at all, but that would simply be bizarro world, and there is enough precedent against it. If APIs did not act as clear boundaries, AGPLv3 software would indeed spread to everything it touches or traverses, like an epidemic. If yours or your customers' lawyers claim otherwise, you/they should get ones with an actual understanding of software and IP law.