And in almost all of those cases, they'd be wrong.
Interestingly, I still have some code in the codebase, which I guess makes sense because I submitted about 680K LOC (note: not all hand-authored, there was a lot of output from automated tools in that) and 2^15 is 32768, so I'd expect to have about 20 lines left, which is actually surprisingly close to accurate (I didn't precisely count, but a quick glance at what I recognized suggested about 200 non-deprecated lines remain in prod). It is not at all the code that I thought would still be there 15 years later, or that I was most proud of. The most durable change appears to be renaming some attributes in a custom templating language that is now deeply embedded in the Search stack, as well as some C++ code that handles how various search options are selected and persisted between queries.
I think this both proves and disproves the original point. Most of your code is temporary. You have no idea which parts of your code is temporary. It's probably not the parts that you wish were temporary, which will almost certainly be made permanent.
When it comes to professional development, I've almost never worked on a codebase less than 10 years old, and it was always [either silently or overtly] understood that the software we are writing is a project that's going to effectively live forever. Or at least until the company is no longer recognizable from what it is today. It just seems wild and unbelievable to me, to go to work at a company and know that your code is going to be compiled, sent off to customers, and then nobody is ever going to touch it again. Where the product is so throwaway that you're going to work on it for about a year and then start another greenfield codebase. Yet there are companies that operate that way!
I've seen mismatch in each direction..
When you get a fatter check because your code break, the incentives are not in favor of good code.