Nothing wrong with easi outs, we used them a lot in aircraft and still do so..... I have lots of the things what they are not good with is rusted in bolts, but as you had copper slipped yours it would be fine, I suspect you didn't have the correct support bracket in place hence why is broke, they are SAE5 bolts so not overly strong but still more than capable of doing the job if supported and bot bolts are tightened correctly.
Cooperman is right about the 1000kg shear capacity, but this becomes a little different when you don't have the supporting bracket and you get a combination of tensile and shear forces, this combination in your case has caused fatigue, in just the same way that Uri Gella breaks spoons.
Part item number 23 in the picture below
The support bracket (part 23 in the picture) was in place, however i do question if the bolt (bolt 24) was tightened enough. By the previous owner it had been replaced with a longer bolt with a nut on it to tighten down onto the bracket, if that makes sense? and i had noticed mid week that that nut had come loose, so if it came loose once, maybe it came loose again, and my "spirited" driving yesterday coupled with the loose nut caused the long bolt to shear?
Either way I have ordered the repair bracket and I will couple that with the thermostat steady next time i order from mini spares and hopefully that will sort me out. I know the correct option would be to drill the sheared bolt off, but not having the tools / time / confidence to that at this time ill go with the repair bracket :)
Thanks all for your input :)