OP, people have confused two different discussions into one subject. To my knowledge they have never said it would break saves that was about a different thing, like moving trees, trash cans, benches etc. or whatever is placed in a common space. One guru explained they don't like to do that since it could break all our saves. That sounds reasonable. Usable objects being added or moved by gurus and patched into your game is what they were talking about early on when they used to hold those discussions.
The other discussion was specifically about swimming in water in TS4 (lakes, oceans etc. in existing worlds) they explained they would have to build an entire new texture for a world like WB. It's all FX. FX is something Sims can't use, touch, enter etc.
Other game worlds are built with above and below sea levels in their terrains. From the get go. When they are designing a game. Even if you never did dig a hole in TS1, it's terrain (world) had a sea level. Later, when they released the ablity to dig ponds in the world view you would see the 'water'. But still Sims couldn't roam the world but it had a sea level. Those ponds were not objects, either.
It's more complicated than I can relate today, with my limited knowledge. All I know is they said no they would probably never patch in swimming into existing worlds, it would be almost impossile since that is all FX. And you have to wonder if it would be cost effective to do that for every world, (since terrain tools were a pet project and not all that useful yet) why they would even try to alter every terrain into a brand new terrain for every world when people will buy a pack where it's limited to a new world. They have nothing to lose by just building one world where this can happen and not wasting time or money trying to satisfy those who would like to see swimming in bodies of water in all the worlds. It's probably never going to happen.