I’m pretty sure having TWWH2 will be well worth it when/if you get the third one. For one, many of the factions on the Mortal Empires (mega-campaign) map will probably require ownership of the prior games. The business model has worked well so far, so doubt they’ll change it.
Edit: Per the FAQ, only owners of all three games get the mortal empires campaign. So yep - totally worth having.