Attack on Titan Season 4 spoilers (not past where you are) Eren only has a few years left until he dies. So they could wait until around the deadline before Armin has to eat Eren to combine all the Titans. But I'm not sure there's much point to this since there aren't any known special powers unlocked by collecting them all. It's just the Founding Titan that has all the busted abilities (if they were usable).
I know it hasn't ever been spoken of up to this point, but I'd just like him to try. I guess it's ultimately more beneficial to have the powers spread among multiple people.