Do we have to subscribe and pay for Origin now monthly to play the Sims?

You don't need the Origin monthly subscription if you already purchased your sims games. You just need to download the origin client and find your Sims game in your game library on your account.

The only reason to get Origin's monthly paid subscription is if you want to try out a lot of different EA games. If the only thing you want is The Sims, then just buy the DLC when it has a big sale going on and you'll save more money that way.