We also did a lot of intentional action to get the resource usage down in the Plasma 5 generation and timeframe.

E.g. the machine we optimized for during at least one or two Plasma dev meetings I remember was the original Pine64 Pinebook, which was a very under-powered device. We had a stack of them to hand to devs. Intentionally as a "if we can get it to fly there, it'll fly anywhere".

So it's not just that we haven't gotten worse, we also did get legitimately better in later releases compared to some of our porkier ones (which also did exist).