> I was under the impression that Ukraine's air defense is just our air defense, just 20 years out of date.

My impression is that Ukraine's air defence is whatever they can get their hands on (and rightly so). This (I think) includes everything from old Soviet and western systems to relatively new western systems. From the fairly-recent essay [0] by General Zaluzhnyi*:

The number of anti-aircraft missile systems was
significantly increased mainly due to Western-made assets, in particular, "Martlet",
"Starstreak", "Javelin", "Piorun", "Mistral", "Stinger", "Grom" man-portable air-defence
systems, "Gepard" self-propelled anti-aircraft guns, "Skynex" air defence gun systems,
"Avenger", "Stormer", "Patriot", "Hawk", "IRIS-T", "NASAMS", "SAMP-T", "Crotale-NG"
air defence systems.

> And I'm also under the impression that what needs defending from isn't going to look like what Russian does now, as Russia is essentially a failed state, using tech that's even older.

Whether or not Russia is a failed state is a very different matter than the current state of Russia's missile/artrillery/drone stockpiles. As I understand it, what Russia does now is not hugely different from what Ukraine does now as far as air attack is concerned (cruise/ballistic missiles, drones, artillery, bombs, etc.), though there are of course differences in the details and the amount of ammunition available, and these change over time. I don't have a non-paywalled link to back this up, here are some (approximate) quotes from a recent episode of a podcast conversation between three defense analyists who seem to be pro-Ukrainian, but mostly not at the expense of accurate analysis [1]:

The Russians do things at scale pretty well. Right, so they're slow to adapt, but when
they get hit once or twice in the face, they do adapt. And then they do things at scale
pretty quickly.

... when we talk about drones, you know FPV, Ukraine was the first to really adapt and
experiment with FPVs, last winter, maybe last fall. They did this before the Russians
did this in any numbers, but ... a lot of production is still on a unit-by-unit basis,
individual guys building stuff in apartments. The Russians waited, but now they are
producing them at scale, and so now on multiple parts of the front line the Russians have
a quantitative advantage in FPVs. And so basically when we talk about the Russian military,
they're slow to adapt, but when they do adapt, they then produce things in large numbers,
pretty quickly, and they become more resilient than some people imagine.

There is a danger of wishcasting. There is a danger, and this has been present from the
start, even before this war, of these stereotypes about the Russians, and maybe some
stereotypes too about the Ukrainians, ... and it's not actually beneficial to the
Ukrainians to have this ... 'Russians are clownshoes' narrative out there.

Defense establishments do not do nuance. They understand that the adversary is either
twelve feet tall or four feet tall, but they are not good at ... understanding that
forces evolve over time, the relative balance between them, the militaries change. You
don't have the same forces fighting in 2023 that you had in 2022, or fighting in the same
way.

* Commander-in-Chief of the Armed Forces of Ukraine

[0] https://infographics.economist.com/2023/ExternalContent/ZALU...

[1] https://warontherocks.com/episode/therussiacontingency/30005...