I was judging by the amount of engagement his Patreon posts get, and it's a lot. Compared to the engagement of some developers who make around $5K a month, his is about triple-and that's without counting Ci-en
I agree, his Patreon income definitely won't be low.
Should be at minimum high 4-digit, quite possibly above, judging by number of likes on his posts etc.
But Patreon isn't secure income, for obvious reasons.
Plus, one can expect somewhere between 30-50% of that to get lost in fees, currency conversions, taxes etc.
Also, Fujino can't go back to China, where you can get jailed for making something NSFW even if it was in the past.
And JP probably won't let him stay permanently until he's lived long enough in the country, which he can probably only do on a work visa for now, which his patreon/ci-en income most likely won't count towards, even if the income as such is high enough.
Anyway, I doubt his income is low, but I also doubt he's dragging out the game just for that.
I suspect it's more like a mix of being tired of SAO, less time and energy due to work/IRL, and trying to cover as many fan wishes as possible before it ends (plus, he presumably needs some time to prepare the next game for a seamless transition).
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Anyway, to be back on topic, from what I've seen of the last releases I think I'll wait for the November release or even the final release before playing again.
It's a little demotivating to have to start a new save (or use someone else's save) every small update.