But now, in 2026, 15 years after the first altcoin was launched, I am a bit disappointed.
Why are you disappointed? Most altcoins were created, because of one reason: mining. If people would be able to mine BTCs directly, from Bitcoin Core, without using any mining pools, and they would receive fractions of satoshis on a regular basis, then they would have no reason to make any new altcoin. Most (if not all) altcoins were made, because mining BTCs became too hard. However, if users could get a block template, mine it, and see it accepted in some P2P subnetwork, then they would never need any altcoins, because that's how any feature can be enabled.
Monero and Grin as privacy coins for example
I think some of these features will appear on BTC, sooner or later. For example in the form of Silent Payments. In general, whole subnetworks could use single UTXOs, and then peg-ins and peg-outs could work directly between one subnetwork, and another. Then, BTC could check the Proof of Work, and a given subnetwork could check all other rules between its own peers. But of course, if there would be no consensus for sidechains, then developers will keep launching completely new chains, just to mine a lot of coins, and sell them for BTCs.
Altcoins with fast block times?
Faster block times are possible, without making any altcoin at all. For example: many mining pools send shares every 30 seconds or so, even though the block time is set to 10 minutes. And P2Pool created a chain of blocks, where they appeared every 30 seconds, and were 20 times easier to produce, while also being compatible with the mainnet. If you need a faster block time, then it can be done, just by introducing fractional confirmations. For smaller payments, it would be sufficient in practice, because the cost of making a Proof of Work share would be bigger, than the transferred amount, so there would be no incentive to cheat.
Privacy coins? Are less scalable.
Well, sidechains, and other second layers, usually have one nice feature, which (by default) is not there, when you have only the first layer: you can remove data, after some time. If you have Lightning Network, then you can finalize things on-chain, and then you can safely remove all historical LN transactions, without harming anyone. In exactly the same way, sidechain history can be discarded periodically, after being settled on-chain. And even all of the history from the first layer can be also trustlessly pruned, if it would be committed to some upper "layer zero".
And smart contract chains suffer from MEV and other vulnerabilities.
If by MEV you mean
Miner Extractable Value, then it is one of the reasons, why there is no consensus to have sidechains on BTC. However, BTC by itself allows you to make such scripts, for example by using OP_SIZE on a DER signature. Then, any user can say "I want to use signature with N bytes or smaller", and any miner can grab this transaction, and move coins anywhere else, by producing a second valid signature. Which means, that if MEV is the problem, then it is already there on BTCs, and it is a matter of putting more coins on such contracts, to see it being exploited.
Does somebody think there is any altcoin project that is, on a whole, "better" than Bitcoin?
No. There are only some good ideas, which should be imported by Bitcoin, where some developers were not skilled or patient enough, to reach consensus, and deploy it on BTC. And then, when they had a choice, to keep talking with people, and convincing them, or just launching their own chain, and making a lot of profits, then they chose the path, which gave them more money, even if it was less beneficial for the users. And then, by mining their own chains, and selling these coins for BTCs, they quickly became wealthy, and stopped caring about improving things further, because they already reached more, than they hoped for.
We need that kind of mindset. That will bring innovation and the result will be "better than Bitcoin".
That kind of mindset requires a lot of patience, and thinking about deploying things, despite it being heavily criticized. If anything like that would ever happen, then it would be just launched by someone, on top of BTC, without asking for permission. Because you can always invent something better, however, existing participants have a strong incentive to keep things unchanged. People are worried, that if something new will be deployed, then BTC will collapse, so it is better to just maintain, what we have, and make as small changes, as possible.
Satoshi created Bitcoin to solve a problem not to fill his own pocket.
This is yet another reason to think, that if something significant is prepared, then it will remain hidden for a long time, and then suddenly it will appear somewhere. Satoshi started working on Bitcoin in 2007, and kept doing it for almost two years, before mining the Genesis Block in 2009. If anyone has a great idea, then for any blockchain-based code, there is a huge incentive to keep testing things secretly, and reveal them later, because once they are deployed, it will be easier to launch an altcoin, than to convince BTC community to introduce any changes.