You propose a solution but you don't specify what even the problem is. "Submarine income" is not easy gil, that perception can only stem from someone who has never tried to set up multiple workshops themselves.
As others have pointed out many times, it takes months of prep and a big pile of gil to get it started. And if you want to get into the big FC numbers, you need multiple accounts for logistical reasons. So why shouldn't people be allowed to do it in your opinion?
Also, people don't do this to sell gil, that would be stupidly ineffiecient. Because when you will inevitebaly get banned, it takes forever to get it up and running again.
And as for why people do it then, i can only speak for myself: it's just fun to see the gil number go up. Started with gathering, then got into crafting to hit the first gil-cap and kept searching for ever more efficient ways of growing that bank. It's just the "natural" preogression of looking for ways of investing your gil to get better returns with the less amount of active management required.
So people that do this at scale are already rich and you aren't really "annoying" them by imposing any kind of restrictions or nerfs on the workshop system, they already made their fortune.
I personally couldn't give 2 shits about it being nerfed at this point. I'll find somehting else to invest gil and time into.
I have 80+ FC's running subs, but i bareley check them anymore because it gets boring over time, so i would even welcome a change to workshops to have a reason to look at other stuff again.
Imo it would be way cooler if they made all the workshop related materials and rewards bound to the FC, so you had an actual reason to work together to make vessels and do voyages, to get the rewards for your members.
But as it stands, it can easily be done by 1 person and all the rewards are worthless because they can be sold on the MB. So all that really is worth doing with the workshop is farming salvage for gil, which is really boring.