I'd take CI over SS any day. They hold so much more heat and cook better....easier to clean, and last a long time with just a little bit of care.
I'm enjoying the SS grates I put in mine, so far. But they are pretty light compared to the original cast grates or some of the rod-stock SS grates you can get or see in ultra-premium grills. I think the mass of the grate matters much more than the material, for the reasons you mentioned. I would have no objection to a set of bare cast grates but you can keep the porcelain-coated versions. When that coating begins to chip off, it's done for IMO. Maybe if you had a way to strip it back to bare cast. The original cast grates in mine were getting thin - but they also were 20+ years old....
I may have found a deal on this on marketplace. I may have also bought it. I don't know why kind of crazy it takes to keep a grill this clean for 10 plus years.
So long as the frame/cabinet is in good shape, that's an awesome score! Looks like they took care of it and scrubbed it down periodically. Might have seen some oven cleaner a time or two as well.
I found a good way to clean my grill, same way they cleaned the Statue of Liberty in NY Soda Blasting. See my pics here. Cleaning Grill grates with Soda Blasting like the Statue of Liberty See video of grill grate blasting here using the Harbor Freight soda blaster. https://m.youtube.com/shorts/17Fi1gDkHDU
I recently used some fine glass material on some solid cast grates. A little too fine, but did the trick.