1) Gil making methods isn't the reason for people hoarding houses.
2) Gil is essentially worthless for people who don't do housing. It is one of the very few gil sinks we have that actually works.
3) 400k per day is pretty average for people who go hard on making gil. I casually make ~150k a day just by touching a few activities. This isn't as insane or game breaking as you think it is. It is also very well known.