Is this promise about future Sims 4 content true?

To be honest I don't know. They said "years" are left but we don't know how many years. And I don't know if they would produce content for both at the same time. I do however, totally understand how you feel because I don't want ts4 to end either!! There is so much more they could still bring to it! I would be thrilled with another 10 years.
Ts5 might actually be a good game but still, that means years of waiting for content all over again.
I still have tons to do in ts4 that would keep me busy for years but I would miss the exciting announcements for ts4. And I know a lot of modders and builders would most likely drop ts4 and move to ts5.