I think the key to this argument was really the statatement "when you can afford them you don't need them" That's the only problem with ronin.
I think the issue there is that they are available specifically when they are in your region. If they were available twice as long that would mean that they move half as often so ultimately you would see even fewer ronin, even if the ones you saw were there a bit longer.
If this were the intended spawning rate maybe you could fix it by having them show up while in regions adjacent to your own. Generally we wouldn't even notice and players wanting to game the system could cast a wide net to catch them- but maybe just amplifying their numbers (in the next patch?) will be enough.