Replace the subs? Replace the monorails? Hell just get a wrecking ball and bring down the castle.Meh.
Something you have to remember: all announcements are made strategically. Disney didn't make many announcements at D23 because... they didn't have to. There was no strategic value in doing so. The parks are doing well, the demand is still sky high, and making announcements now probably didn't make sense.
On the other hand, bluesky concept work is always going on at WDI. They create ideas every single day. Some get a little further than others, but the vast majority of them die off before becoming reality. To be honest, "New Tomorrowland for Disneyland" is one idea that seems omnipresent.
Looking at history: it was nearly 15 years before the initial ideations and the opening of Tomorrowland 98. Some of those ideas got so far that concept art was published (Montana Tomorrowland), and other ideas started having actual job orders for work (Tomorrowland 2055). So even if something gets announced, it isn't a guarantee that the whole project gets completed.
The one thing working toward the eventual announcement of a Tomorrowland redo is that the Subs and Monorail still need to be replaced.