guarantees fees for miners after the block reward era.
You mean the block subsidy. Theoretically, there will always be a block reward, which will later on come exclusively from transaction fees.
The negatives are its similarities to dystopic negative interest rates, and a dampening effect on bitcoin's scarcity. Whether or not this is a good idea, I want to implement it and test it out for fun.
You're doing more harm than good that way. If you want network sustainability, you shouldn't rely on the inactivity of the holders. If there are no transactions on the base layer, ergo no activity, I don't find a reason to have sustainability at all. If there will be demand for bitcoin, there will be transactions, and therefore sustainability.
I like the idea personally because I think everything in this world must be paid for, and coins being held forever are getting free security from the fees of new transactions, creating little incentive to move these coins and make renewed contributions to the network's security.
But, the holders did pay for their security. They pay it when they sent their coins to the cold storage. The people who make transactions regularly are those that should pay more, because they take up more space.