Everybody has a different need for heaters.
Most people working with tropical fish and plants live in the Tropics and they don't use heaters.
Well I live on the west coast of the east coast. AKA Tampa FL. Tampa Airport's largest export is tropical fishWell north of that but I work in Tampa. Those farms are about 20 miles south of Tampa. Segrest is there. Many of our society members work for, or have worked for the farms here. Contrary to what you may think it gets cold here. Back in the 80s we were as low as 20. And rolling blackouts. Yet I lost no fish. The farmers did have great losses in the ponds however.
This winter the fish house was as cold as 65. I did put a space heater in there and kept it at 72. A couple of my more sensitive tanks I did toss a heater in but none of the rest.
As I said in my OP the farms here don't use heaters. Unless it stays cold for an extended period we won't lose fish.
Who really wants us using heater? Yuup. Heater manufacturers. You fish will do fine at 72-74.
Mollys, many cichlids, many other "tropical" fish can do fine at low temps. How do I know for sure? Because some morons let tropicals go into waterways, ponds, lakes and do fine. So much so that a few have become invasive.
IF one of those heaters failed (on), the other(s) would cycle less. IF one failed (off), the others would cycle more. In neither case, would temperatures endanger the living things.
For example, you calculate that you need a 400w heater for your 100 gallon tank. Plug four ugly 100w heaters in there. If one fails the others compensate accordingly.
Like most things in the hobby, your best bet to keep a close eye on the water, fish, and plants while maintaining your equipment! But ... has anybody else tried this?
I've been espousing multiple lower watt heaters for decades. Well before the internet.
Not much to do maintenance wise with heaters. Any heater used long enough will fail. Although I do still have a 1980s Supreme heater I use in one tank.