That's not entirely true. It really depends whether the original design's transition from subsidy to fees works as intended. The recent uptick in the fee portion of block reward is a good sign, but we still have very limited data.
I would say that is mostly due to the limitations of Bitcoin as of now. This could be sustainable though, if the transaction volume continues to increase and miners are able to accomodate more transactions per block.
The premise that fees will replace subsidy without fundamentally threatening the network's security is still untested. It's theoretically possible that waning future block rewards could lead to hash rate drops that throw Bitcoin's byzantine fault tolerance in question. Severe enough drops in hash rate could allow older generations of hardware to come back online, opening up the possibility of massive block reorganizations -- and therefore utter unreliability as a value/monetary system.
Massive block reorgs would most likely happen if someone decides to execute an attack on Bitcoin. Old and obsolete ASICs are still able to come back as of now. There isn't any relation to older ASICs coming online, as far as the security of the network is concerned. If you're going to attack the network, might as well as use the most efficient ASICs out there, no need for old ASICs.
If this were to happen, a perpetually low but predictable inflation rate -- which guarantees some mining security incentive (fees cannot do that unless they are mandatory and sufficiently high) -- would look very attractive in comparison. That's your economic incentive.
I'm really bad at economics, so forgive me if I misunderstood anything. Isn't the additional coin going to affect the market? If its too small, it wouldn't make a difference to the miner.
The main thing that attract most of us is that Bitcoin has a fixed and transparent coin supply. I doubt most people would support breaking the basic feature that defines Bitcoin.
You can but no one will take those modified and increased supply coin, just like those forked coins.
So, let me ask you again. Can you technically increase the supply of coins? Even if its a hard fork? It seems like what you meant is that the rules of the network cannot be changed since the genesis block, while changes are still being made. It is possible with consensus.