I've always been impressed by the backward compatibility of Notes. Late in 2008 I re-met an old client who was still running a Notes database I had built in Notes R1 (yes, R1) some 20 years ago. His Domino server had moved up to R7 by then but the app was still chugging along storing client records and printing sales summaries. It sounded like a story for the IBM marketing machine except for the fact that he was in the process of replacing the system with a purpose-built CRM product.
Then there's the customer who wants me to paste a quick-and-dirty XPage front end onto a database he had downloaded from Open NTF. In the end I advised against that approach since it would take a couple of weeks to reverse-engineer and rebuild the undocumented lotusscript agents buried within the existing Forms.
We can also consider one Business Partner's six figure commercial software development project that was originally designed with R7 Lotusscript and is now being retrofitted with XPages to make it more appealing to new customers. I know there are some good people working on that project but maybe they can't see the forest for the trees - new clothes do not make a new man in the world of software.
So how far can you stretch the rubber-band? Sure you can take an existing DB, upgrade the ODS and whack in some XPages over the weekend but is that a good thing? Aren't we just creating the 21st Century equivalent of 1980's COBOL spaghetti code?
Maybe Microsoft does have some benefits with their 'rip and replace' approach to technology. Applications developed with their tools may be over-budget and over-schedule but at least the code is fresh.