In some ways the Su-35 is like the JAS 39E - the outside looks the same but the inside is very different. It would be interesting to know how many original Su-27 parts are in there, and my guess would be "not very many".
But if you have a good design and your fighter requirements don't call for higher speeds, greater range and payload, or anything that would drive you to a new platform, why not?
Obviously if you want stealth, you change the platform. But as we're seeing it's difficult. Just how difficult, we are seeing as we speak, as DOT&E Gilmore prepares to unload on the JSF before the Senate.
