I get your point that it’s convenient to be able to blindly trust a slab, but in older houses you often have slabs that are too cracked, have settling underneat

I get your point that it’s convenient to be able to blindly trust a slab, but in older houses you often have slabs that are too cracked, have settling underneath, or are otherwise unsure or suspect enough to blindly assume the “concrete will behave like concrete.” In those cases it’s pretty straight forward to just cut/demo out section of the slab, dig a new footing or prep for new slab in that area, and then reinforce as spec’d for the intended load. E.g. car lift in older garage. Even w/ new(er) construction I wouldn’t be trusting it to handle any sort of outlier use case without testing, or at least having the original blueprints w/ specs for the slab.