This was asked and answered before and I'm not sure what you expect to see here. We can write any science fiction essay we want and I would be surprised if reality will not beat at least some of them.

I like to think that bitcoin will be used as reserve by central banks, only large transactions will go on-chain and the bulk will happen on alternative channels (LN and similar), also that even states or banks will mine (for a good hash rate and also for including their own transactions into the mined blocks).

I think the heirs will be laughing their asses out in 2140 looking back at the 101 million topics about the reward and the same answers being posted again and again. I bet their first impression would be:
"These idiots invented Bitcoin"?


lol...

Well, I don't think there is much to about this to discuss, and besides, has anyone mentioned that this topic has been discussed here before? , on several occasions, anyways, when every bitcoins are mined, making money from bitcoin mining will become only based on transactions fees collected from from those using the bitcoin network, at least, mining bitcoin could become easier and will no longer consume electricity as it does now I believe..

And another thing worthy enough to note is that by 2140, most of us are no longer here must have joined our creator, so whatever be the outcome of this development by then will be solely based on decisions made by that generation by then, so personally do not see a need to be discussing this as we can not develop bitcoin alone, the future generations has a part to play in the future of bitcoin, even when all bitcoin have been mined.

so.. you want bitcoin to become a government network.. and users thrown over to silly subnetworks that have flaws and no security.. um no.. but ill just say: goodbye, so long and thanks for the fish.. if thats your guide to the crypto galaxy

id rather us normal folk get to use the secure bitcoin network and its the banks that do stupid stuff charging people middle men fees on their silly corporate designed subnetworks that require middle-men to route payments.

..
for anyone that thinks that bitcoin should be limited to say 6000tx a block with a $30 fee per tx to give pools $180k. a block are thinking rather narrow mindedly.. but in a narrowminded mindset of what benefits central banks/corporate banks.. not users/customers.
so lets use FIAT wire and a corporate payment system as an example..
if it costs users $30 to make a wire transfer no one will want to use it. even if its just to lock-in to another subnetwork. people wont even want to pay $60 a month to unlock-relock value into different payment system offering cheap fees once locked in..

imagine it like paypal saying they operate faster than bank wire transfer and only charge 1cent. but there is a $60 monthly fee to get in-out of paypal to audit/settle account. people would avoid paypal and bank wire transfers and find a different currency altogether

the solution is not to make wire transfers expensive and hope paypal win customers whilst keeping people semi linked to 'dollar'. becasue people would end up just avoiding the dollar and instead convert to pound or lira.. the solution is to make dollar wire transfers better. and then have the silly niche sub services for thinks like starbucks loyalty cards and apple giftcards for payments within them systems

...

by having more transactions per block means each user pays less
EG the (if lean) possibility of 4200tx a block in 2014-2016(1mb allotment)... means (if lean) we could have had 16800 tx in 2017-2023 without breaking the 4mb accepted allotment.. and by 2140 knowing storage space for terrabytes would be like 1mb floppy disk prices of 1999.. we could have more then 16k tx per block. meaning users pay pennies not dozens of dollars

we are no longer in the pre-millenium era where 1mb cost $1
and in 2140 gigabytes wont be more then pennies

No, contingencies for this should be in place long before this happens. As in, having a constant stream of transactions with a steady fee. Ordinals and BRC20 were that a few months ago, but now nobody even talks about them (for good reason though).

It might annoy the users of Layer 1, but increased transaction fees, up to a point, will be how the miners survive without the block subsidy.

ordinals bloated up the blockspace but paid less than the average fee per byte.. so not a test of the future expectation.. its actually the opposite..

increasing fees per user is not the solution either. the real solution is actually increasing the users. to give a better total block fee.. that the difference

Sometimes we want to shape the future by our actions today. But the future is unpredictable because it comes with new threats and opportunities. Technological the future generation will be better, I am sure they can handle bitcoin.

High transaction fees will discourage people from using bitcoin. It will end up becoming a currency for only people that can afford the transaction fees. I was thinking that cheap transaction fees will increase adoption and when many people use the network miners will make more money from the transaction fees.

If the youngest person here is 10 years old, in the year 2140 he will not be able to comprehend what he is saying, and therefore we all do not care what happens because we will not be present.
Science is developing rapidly, and it has become impossible to predict 20 years ahead. more than 100 years is an open possibility for science fiction and thrillers, not for facts and logic. Look how our lives have changed since the year 2000, and how it could be after 100 years.

Unfortunately not a causation. The increase always coincides with a rise in the popularity of Bitcoin and the effects of the halving are often either negligible or difficult to measure. I have my doubts that the halving would actually cause a significant rise in the value of Bitcoin, if any.

Everyone validates transaction, and there is no actual puzzles being solved. The issue is when the cost for an adversary to execute an attack becomes too low.