Does the "Legacy Edition" of the Sims 4 confirm we are at *least* halfway through The Sims 4's life?

Well they have info about the Olympus project build going back to 2008 when it was started - so technically Sims 4 was worked on since 2008, but they changed direction in early 2011/2012 and started rebuilding the engine, changing it for one - to not be online. The devs were not happy with it even before the disaster that was Sim City 2013. So that sort of tells you 2 and a half years is not enough time for a good base game as you saw how Sims 4 started....

Sims 3 base game was started well before Sims 2 Open for Business came out - Tim Letourneau let that slip at the 2005 Gamescom the year they were showing us Open for Business.

Will Wright stated they started Sims 2 as soon as Sims 1 base game was shipped.