I work for a company overseeing network design of a large industrial system. For our purposes, it's a backbone network that other stakeholders have their own L3VPNs defined on. It is a network with its own private cabling infrastructure, and as such is not connected to the internet or any public infrastructure except on a few specific VLANs through a customer firewall. Everything is IPv4.
In reviewing their IP addressing, I've found that the contractor in charge of the design has assigned public ranges to every subnet. We raised this issue to them, and they have readdressed the network to be within the private RFC1918 range. We are definitely making sure that the VLANs accessing the internet are on private address space. However, since they are determining the address space for other stakeholders in the project, there is an issue where those stakeholders now have to make significant changes to their equipment that was already commissioned/installed in difficult locations(there's a lot of endpoints) and there is now some question of whether we can "get away with" having those subnets remain within these public ranges.
To be clear, I know this is bad practice. I'd rather bite the bullet and readdress everything now but it's not completely up to me.
However, if we can guarantee that the devices in question will never have to access the public internet, are there any other issues that could arise?