The big savings would be in longer lived electronic devices. A sizable portion of digital electronics ultimately fail because the capacitors in their power supplies exceed their designed lifetime. Electrolytic capacitors (ticking time bombs) are cheap and compact for the capacity so they are used for the energy storage required in single phase systems, but if designing a three phase power supply less storage is required and maybe manufacturers wouldn't have to optimize the smaller, capacitors with electrolytics. I've had a 4k TV and a rather expensive musical instrument amp fail for capacitors this year, about $3000 of gear there. Looking back further there are a couple of professional powered sound reinforcement speakers (twice), an iMac G5 (three times). It's probably safe to say over the decades I've had $10k of electronics fail prematurely from electrolytic capacitor failure. These are all items which were not obsolete or worn out. Scale that up from one person to worldwide and it's easily in the hundreds of billions (10⁹) of dollars which could be saved.
The problem is you can't get there from where we are. No one wants to drag more wires to residential houses, no one wants to shell out >$1k for a household phase converter and then rewire their house. You won't convince people to add three phase to a new house because it might be the future. And you won't convince the semiconductor houses to make a line of three phase power supply controller chips just because, well, maybe someone might use it.
We're probably trapped at a suboptimal local maximum.
Edit: In response to atoav's comment I found IEC 60309 and its extra low voltage variant which is a 42mm (1 2.3") 3 phase plug and connector in two compatible sizes good for 1.3kW and 2.6kW. Also, that Germans and Swedes did the engineering first and then built an electrical system and they get 480V 3 phase at 16A to their cooktops and stoves through a plug for a whopping 13kW!