One thing to note is that you absolutely can fully replace a legacy codebase a piece at a time, the big risk is when you have a completely incompatible “rewritt

One thing to note is that you absolutely can fully replace a legacy codebase a piece at a time, the big risk is when you have a completely incompatible “rewritten version” of the product that you will cut over to at some later time. If you do something like build a facade in front of the old service, and gradually move functionality into the new layer, this can be seamless for users. But of course, this doesn’t give you the ability to revisit product functionality, like the Basecamp rewrites. I think those are really great examples of how to continue to innovate and experiment, while keeping your old customers happy, and the key point there would be “rewrites can be done safely if you commit to keeping older versions around”. Basically treating the versions as distinct products, instead of the rewrite replacing the old version.