At some stage, "improvements" become nothing more than marketing buzzwords.

I'd argue one reason ICE are "stagnant" are because they're "good enough" and any potential improvements required expensive R&D and manufacturing changes, for results that purchasers won't change their buying decision for. Maybe Toyata could make an ICE for the Camry that was 5% more efficient, but few people who were about to buy someone else's equivalent car will choose a Camry instead based on such a marginal improvement.

I think phones are a good current example of this. I have felt zero need to upgrade from my iPhone 13, because the "improvements" since it was new are of zero interest or value to me. I'm quite likely to do a battery replacement on this one instead of upgrading to a new iPhone any time soon. (And the only reason I bought the iPhone13 was to get the backside lidar, I was perfectly happy with the XR I used before that.)

Isn't that a different way of saying the same thing?

There's no denying that ICE Engines have been around for a long time, relative to EVs. A mature technology means that any low-hanging fruit for performance improvements was found a long time ago. Remaining gains require the expensive R&D for marginal improvements that you mention.

It's not that they're "good enough" - any consumer who can do the sums would prefer a 20% better price/performance, always. It's just that such improvements are not there to be had in ICE vehicles. There is no rush to improve them, as you said.

And therefor the performance of a new ICE vehicle, 5 year old vehicle or 10 year old vehicle is well-understood, predictable. Both in terms of what the tech was when it was built, and how it has aged. EVs are less so in both of these dimensions. e.g. I would have reasonable confidence in the durability of an EV battery for a 2025 model, but much less so for a 2015 model. They're just not the same.