I've used Soft Scrub for years on the sinks in this house. But the single deep sink in the kitchen already had the "sandpaper porcelain" finish. The one in the patio kitchen was in much better shape, but too much use of Zud fixed that (I think). Zud is evil - if it's left in place on porcelain too long, it will actually eat through the finish all the way down to metal. That didn't happen in the patio sink, but the finish on the bottom is rougher than on the sides, and the only thing I can think that happened was Zud.
Soft Scrub is not supposed to wear away porcelain enamel. It contains no diatomaceous earth (unlike regular cleansers), just a sort of chalk not unlike Bon Ami (I think). The bathroom sinks where I've also been using it still sparkle and have a fine finish. It does contain chlorine bleach, which is what is needed to remove food stains from the worn kitchen sinks.
I suspect that there are varying qualities of porcelain sinks. Not just the use of cast iron vs steel as the base, but also in the thickness and composition of the pigmented layer as well as the glass layer.
I'm not too concerned about the condition of the patio sink. I use that for a lot of non-food related purposes, like washing latex paint brushes, etc. By accident I also discovered that a sodium sulfite solution will depigment the porcelain. The glass layer is still intact, and not depressed, but you can see through it down to the gray metal layer. Fascinating. I used sodium bisulfite (also known as hypo) as a chlorine remover for water used to top up the adjacent outdoor fish pond. I guess I left a spoon or something with some chemical on it in the sink too long. C'est la vie. If I were to replace it, the replacement would probably be stainless and a couple inches deeper than the existing sink.
The kitchen sink I'd like to replace, but it's got tile over the edges (looks like an undermount affair) and I'd probably have to break the tile to replace the sink. It's that off-white six in square tile with dark brown wide grouting that was popular several decades ago. I have some extra pieces but I don't think it's enough or of the right profile to replace all the tiles ringing the sink.
As for double sinks... they are useful if you do a lot of hand washing of dishes, I suppose. I like that the patio sink is a single, though. It's about 27x17x5. I like the long length, which allows for washing of many larger items. But the depth is a bit lacking.