I'm going to meta-level with you:

1. No one is saying that the inventions of 1909-1959 were not amazing. Yes, the jet engine is fantastic. Flying is bonkers. Antibiotics, great. No one is seriously arguing that these are not spectacular inventions that transformed humanity. You don't need to keep repeating the argument that 1909-1959 was great, by listing great things from then, because no one is arguing against that.

2. I (at least) am making an argument about the proof you offer that progress is slowing, I am saying it is weak proof. I am doing so in two ways:

(A) by pointing out that the ways you are arguing 1909-1959 things are great can also be applied to post-1959 things, and the ways you are arguing post-1959 things aren't great can also be applied to things from 1909-1959. This argument looks like this: jets are great compared with props in the same way that iPhones are great compared with ENIAC. So why is the former transformation categorically different from the latter?

You are interpreting A as me saying that things from 1909-1959 weren't great. I'm not saying that. No one is saying that. You are saying that my examples are absurd: yes, that is the whole point of this argument. It's absurd to say that jets are not great because we already had props, just as it's absurd to say that iPhones are not great because we had ENIAC (or whatever appropriate computer from 1950 you're thinking of).

(B) by pointing out that it is really hard to know the impact of many really amazing, crazy foundational things until 50 years after the fact. Not all things, of course, which is why this thread is full of examples and counterexamples. People being optimistic or pessimistic about tech in 1969 is irrelevant -- if I found someone who in 1969 thought that 1919-1969 was lame compared with 1869-1919, would that convince you of anything? No, because it's irrelevant. The whole Kelvin thing is apparently apocryphal anyway.

With that out of the way, you ask: what would be enough evidence to convince me that progress is slowing?

First, one problem is that we'd need a definition of progress. "Rate of invention of things Scott thinks are obviously amazing with 50 years of hindsight"--aka your list---is not a great definition because we don't have 50 years of hindsight on 2009. See point (B) above.

Other people have looked at counts of patents, per-capita research papers, etc., all of which continue to accelerate. But these are all also problematic, because these things can be gamed and end up correlated with other things that are irrelevant to that ineffable "actual progress" we're both arguing around.

We could go with lives saved, by which measure 1959-2009 looks amazing except that most of those lives aren't in the USA or Europe, compared with 1909-1959.

In another post, you say (paraphrasing, but I think I got it) "things that give a society that possesses them a guaranteed win in a war over societies that don't" but this is a very pre-atom-bomb type of definition. At this point, the atom bomb is that thing for everyone all the time, so it makes sense that societies don't prioritize weapons like that anymore: you don't need them! We already have the bomb. Who needs a second world-annihilating type of device? So technological progress doesn't look like this anymore, post-bomb. (Even the things you list as examples of post-1969 definitely don't fit. Coding theory feels like a stretch here IMO.) I speculate that it's because societies tend to fight more in economic terms today than physical terms. So we get information-related inventions and financing inventions, not physical-dominance or energy-extraction inventions. But I digress.

I don't have a good definition of progress. But I'd love to come up with one that fits.

Second, without a definition, things are tricky, but I tend to look at proxy measures like productivity. Technology lets people do more, so, how much can people do per unit time? The rate of productivity growth has been slowing a bunch since 1970-1980. Another proxy measure you could look at is health outcomes: life expectancy at birth (reflecting childhood diseases, mostly) has basically been stagnant since 1980.

These proxy measures are what make me think that progress is slowing. But it could be the proxy measures losing their proxy-value instead.

I welcome a new definition of progress from you to argue about.