2020 new retail expansion
2021 TBC
2022 new retail expansion
2023 Wotlk
This makes the most business sense, but I am sure they will introduce new server for the expansions and keep the old ones seperate/merged.
This gives people plenty of time to enjoy the content and if they are bored with one version, they might visit the other after the expansion launched.
What their plans are beyond wotlk I can't tell. If the rumors are true that there is another major world alteration planned in a future retail expansion, then even Cataclysm servers might be attractive to "visit the first alteration".
They could also merge cataclysm and MOP or skip cataclysm stages altogether and go directly for MOP's state.