Salesforce deserve credit for being continuously online as a cloud service since about 1999, and since then regularly rolling out updates and new features to the original codebase without knocking anyone offline. And obviously it has expanded massively since '99 to become the paas behemoth it is today. Think about how brittle code gets after just a few years of incremental updates. Salesforce must have the foundations pretty solidly nailed down to be able to keep expanding the codebase for 20 years without a 'sod it lets just stop and rewrite everything' moment.