That is something false the dev team fed you with.
You can, in fact keep adding infinitly to a system.
Take the PvP talent system. They could keep adding more each expansion, but only let you choose 3. There are other systems too. For example the new skills can be upgrades of old skills with new or different side effects and/or animations.
Basically, there are ways.
It's more like, they want these systems cause they think they can sell expansions with them by making them a selling point.
I think this method won't work for much longer though.