I'm going to vote no on this.
You stated that having this 'upkeep' would help against house flippers and it's been pointed out that no, it would not. House flippers wouldn't keep the house long enough to trigger the cost of upkeep or would simply pay a large sum of upkeep up front and then add that cost to the total of sale.
You stated that the cost would be negligible, like paying for armor repairs, so that it didn't impact anyone too much. In that case it is totally worthless for the purpose of being a house flipping deterrent and it's worthless as a gil/currency sink because it wouldn't siphon enough out of the system (you're talking sub 100k per month numbers at that point).
You stated that it's good for folks who are planning trips or for whom life gets in the way. Sure, if you're planning to be gone longer than 45 days then it might be helpful to pay ahead and pause the timer but for those who find life getting in the way...chances are there was an incident that cut them off suddenly in which case oh no! Suddenly they don't have 45 days to get back online, they only have until their next payment is due. Better hope they weren't doing month to month!
You stated it would prevent house hoarding. No, not at all. You think the people who made enough money to afford multiple houses in the first place wouldn't be able to afford multiple house upkeeps (especially if they're as cheap as you say they should be)? The answer is no. They'd still have all their houses, this would stop nobody.
I'm also glad that you know that all my currencies are unused. That I don't turn any of my beast tribe currencies in for supplies. That I don't turn my GC seals in for ventures and crafting mats. That I don't turn poetics or hunt seals into various items/gear/trinkets. That I have nothing left to buy with MGP. Not everyone is sitting on this wealth of unused things because, surprise, if they don't have a use for the currency most folks stop doing the activities that give it. You're telling people to go out and do needless busywork. I'm sorry if I sound rude or harsh but I haven't seen you actually list any pros to this system beyond bypassing the demo timer, that seems to be the only benefit of an otherwise flawed, cumbersome, and unrealistic system. Thanks, I'd rather log in every 45 days.