From the two or three decades ago? Did you build that one yourself and messed up the power manangement? 
Our last HA Server with a CPU now almost 10 years old actually had a lower power consumption than a Raspberry while still having a better performance. The thing was even smaller than a PI and doesn’t rely on a sd card for storage and was conveniently powered via microUSB and
This thing like most x86/x64 often have a better power management than SBC’s. For example a Raspberry Pi 4 is idling with almost 3W while our device was happy with just 1W 
Correct, and the Raspberry Pi 4 was/is quite bad in this scenario! Worst than a random Intel CPU from the last decade 
Correct, people buying a Raspberry thinking it is low power and ending up with a energy bill 3 times higher than if they would have used a old device from the drawer 
Think of the following: Demanding task coming in your Pi 4 is blocked for couple of seconds/minutes because of 100% cpu usage. Another device (maybe 10 years old) with a 7x higher performance doesn’t need the 100% usage and allows to use the HA web ui all the time while your Raspberry craps out and is unresponsive 
Specification comparison:
| Processor | Intel Atom Z3735F | Broadcom BCM2711 | ||||||
|---|---|---|---|---|---|---|---|---|
| Market (main) | Notebook | Single-board computer | ||||||
| ISA | x86-64 (64 bit) | ARMv8-A (64-bit) | ||||||
| Release date | Q1 2014 | Q2 2019 | ||||||
| Lithography | 22 nm | 28 nm | ||||||
| Cores | 4 | 4 | ||||||
| Threads | 4 | 4 | ||||||
| High performance cores | 4 Cores 4 Threads @ 1.33 / 1.83 GHz | 4x ARM Cortex-A72 @ 1.5 GHz | ||||||
| TDP | 2 W | 8 W | ||||||
| (Multi-core / watt performance) Performance / watt ratio | 938 pts / W | 128 pts / W |
As mentioned it is not the same device 
No? Other than a SBC the thing is complete and works out of the box as unlike berries it comes with case, cooling, flash, rtc, psu 
If you are REALLY interested in saving power you wouldn’t have bought/used a Raspberry Pi 4 in the very beginning 
Benchmarks indeed are the opposite of real life scenarios. Little more than just a grain of salt necessary
Well, it does to some extended. As higher the performance per Watt is as lower is the power consumption. Why is that? Because the device with a higher performance/Watt ratio is more efficient
See the table above? This 10 year cheap atom CPU has a TDP of 2W compared to 8W for the Raspberry SOC. Bottom line the raspberry still performs worse while producing 4 times the heat which is just wasted energy 
The thing was even capable running of 5V 1A PSU (5W max output without the additional losses in the cable!) - only downside was it never never reached it’s turbo frequence - still was it was running stable. 