The problem was the employees wanting to put their personal devices on the corporate network to surf the web.
You never explained why exactly this was a problem. Their devices couldn't possibly be a security risk if your internal servers and networks were set up properly.
Employees were unwilling to accept that there's no good reason for their personal crap being attached to the network.
Maybe they weren't willing to accept that because it's not really true and you are just stating it like a fact. There's lots of good reasons employees would benefit from guest network/internet access: Wi-Fi calling, not getting any cell signal at all in buildings like Casinos and some resorts, better battery life on their devices, not needing to use valuable mobile data while at work, etc.
From a security standpoint letting employees onto the guest network is fine. It makes their work satisfaction higher and costs little to nothing for the business. Maybe you have a policy of not wanting employees to use their devices at work (like in the old days where businesses didn't want employees using phones for personal calls) but that seems more like a political move than having anything to do with security which was the topic.