Where do you think the words "merkle-sum-trees" came from? :)
> Unless you do some crazy ugly hack like creating a separate overlay merkle tree with its root being output 1 of the coinbase, that's a hard-forking protocol change.
No crazy hack is required. You just include a commitment to a merkle-sum-tree tree of transaction values, along with the UTXO commitment. It doesn't have to commit to transactions, it's just a tree of values. There is no loss of efficiency, you don't even have to signal the data normally since all full nodes already have it.
> As for changes actually being implemented, to be honest I haven't seen anything actually substantial since P2SH
Well thats a step back from the position you took above that it can't change at all ever. Now the changes have been not substantial and not often enough. ::shrugs::
> that would benefit everyone
It's highly debatable that it would benefit everyone now, we're certantly not up against the limit. People are still using the blockchain in very inefficient ways, and the ecosystem of tools to increase efficiency hasn't developed yet. At the same time the count of full nodes is falling— increasing the cost of running one right now may not be a good strategy.
> so I think it's unlikely that Bitcoin will develop second-layer scalability protocols first
I don't have any interest in being first. I'd much better have a well designed and considered approach. Unfortunately, so far, none of the alt-systems— even ones which raised millions of dollars of funding— have developed anything that turned out to be useful to implement in Bitcoin. Maybe that will change.