SpiritedSim No one knows for sure, but what we DO know is that your content should continue to work once you've installed it, even if the Store site gets taken down and nothing new can be installed. And it's unlikely that EA would remove Sims 3 overall given that it's a single-player game, aside from the few online integrations that aren't all that meaningful to gameplay and could be disabled separately. The Sims 2 Ultimate Collection still works for people that already own it even though it hasn't been sold or given away since 2018.
The games that get delisted and removed are those that have online services that cost more money than they're worth. Sims Mobile and The Simpsons Tapped Out and Anthem for mobile and the Lord of the Rings mobile game only worked online. Anthem could have been offline in theory but wasn't built for that to work, and switching over would have taken more development resources that apparently weren't worth the effort in the end. I can't think of an example of an EA game that can be played offline that has been removed from the libraries of people that own it.
Still, there's always a risk, and my suggestion would be to invest as little money as possible to get the content you really want. Use Complete Your Set and the Daily Deal sales to your advantage; if you want more info on how this works, you can ask in this thread or in the DD master thread here:
Daily Deal (DD) 2018 Rotation List and Sales Help | EA Forums - 1821670