If we assume these are real concerns for Bitcoin, is BIP300 the only answer?
Or are we trying to make BIP300 be all things for all people
I only want it to be one thing for some people. Not all people will want to use any new softfork. Drivechain would only be for people who trust miners to hold their coins while they use a sidechain.
Regarding privacy, does Lightning and other L2 not solve this?
They definitely help but I don't think lightning solves privacy. Widespread use of coinjoin probably solves it, but coinjoins are large transactions and they take up a lot of blockspace. Some people like to do them on sidechains, and some people would like to do that but they don't trust the sidechains that currently exist. I'd like to help them coinjoin on a sidechain they trust.
There will always be enough revenue for miners to secure the network
I agree
Would you not say that most of the world will be onboarded via Lightning as their 'bank account'
I don't think that is likely yet, not until lightning's reliability improves. (For me, as a daily user of lightning, about 10% of my transactions fail.) One of my hopes and dreams for lightning development is that various attempts to improve the network may lead to the discovery of scaling technology that is even better than lightning. If that happens, maybe that will onboard most of the world. But with lightning working the way it does right now, it is pretty unreliable as a payment method, at least in my experience, and therefore I don't think most of the world would like it very much if they tried to use it. I think they'd even prefer to go back to visa, if they had to use lightning as it exists right now. But that just means there are opportunities to improve lightning! It's gotten so much better over the years and I think it will continue to improve.
Do you have any specific arguments that can only be solved by drivechains?
Not "only," no. I think there are usually multiple approaches to solve problems in bitcoin and several of them can work together to solve its remaining issues. Drivechain is one thing that helps us solve some problems. It helps with scalability, it helps with privacy, it helps with new script functions, but it's not a panacaea for everything, and I don't think it's a perfect, final solution for the things it helps with. But let not the perfect be the enemy of the good. Drivechain helps make better sidechains, it brings more fee revenue to miners, and it does not harm me or (hopefully) anyone else. That's good enough for me to want it.
Could you clarify for me please, are sidechains currently possible at all? Does Drivechain usher them in, or does it simply make it easier?
Sidechains are currently possible, current examples include liquid, rootstock, and namecoin. Drivechain lets us do them better.