3 sats/vb must mean that not so many people are inscribing stuff into the blockchain, right?
Yes, but it can always change. The problem is still not fixed. Another example: testnet4, at the time of writing, has almost empty blocks, and 1 sat/vB is sufficient. However, by having just 10k tBTC (which is not that hard to get), you can fill mempools with 1 TB of transactions, waiting to be confirmed, at any moment. This is more than 600 GB on mainnet, produced in the last 15 years.
Which means, that yes, here and now, you can see peace and quiet. Also because if you put too much spam, then you can undermine your own wealth, because regular users can they say "there is too much spam, let's try something else". And those kinds of things can negatively impact the current price, and even hurt some spammers (because when Bitcoin falls, then many altcoins can fall much lower, and even became abandoned).
Does this mean that transaction fees need to be expensive?
No, because you can have a single transaction, serving thousands of users. In general, if we would have commitment-based transactions, then every 10 minutes, every mempool will be cleared into zero, no matter how big it would be. Because then, adding a new public key, with a new amount, would be simplified just into tweaking a single commitment public key, in a single transaction. And if you have a tree of keys and amounts, then you can update it logarithmically (in a similar way, how SPV proofs work). Which means, that if you have correct signatures, then you don't even have to download those parts of the tree, which you are not interested in. You can just audit them, and check if no coins are created out of thin air. For example: you can see, that the root of the tree has 21 million coins, the left branch has 1 million, and the right branch has 20 millions. If you are in the left branch, then you don't have to download the right branch at all: you just verify a signature, and it is sufficient to know, that those 20 million coins are properly signed, no matter how many users are there.
Therefore, with ~3000 regular transactions per block, to cover 3 BTC (which is currently the block reward), each one has to pay 0.01 BTC
Almost there. "0.01 BTC * 3000 = 30.00 BTC".
Hopefully, we will need less than 3 BTC per block.
I guess 0.01 BTC per block may be sufficient. Because keeping 1 sat/vB means 0.01 BTC per 1 MB. So, rewards in range 0.01-0.04 BTC are guaranteed to happen, if 1 sat/vB limit will never be lowered, and if blocks will be always full.
we can violate the hard cap, and add tail emission
1. Adding "tail supply" is not that easy. Each and every overprinted satoshi may be easily burned by those, who care about keeping 21 million coins limit.
2. You don't have to violate the hard cap. BTC domination of 50% is the same situation as 100% domination with 42 million coins. If some altcoin create new coins, it simply violates 21 million coins limit.
