Depends on what you are trying to do. I had this conversation recently at my LBS when I bought a used Trek 930 mountain bike from them. It was built up with new SRAM Attack 8 speed shifters and new Alivio RD. FD is the original Shimano STX from the 90s. I spoke to them about upgrading to Deore shifters and derailleurs front and back, or SRAM equivalent. Bike guy suggested just riding it first. He was right. While it looks plasticky, it shifts as good as anything I own, including the Tiagra I have on my road bike.

Deore is higher on the Shimano heirarchy. If you can afford it, get a bike with Deore components. But if you can't, modern Altus/Acera might be equivalent performance wise to older Deore.

Now, if you ride in exceptionally hard conditions, like snow, rain, mud, and dirt, your derailleurs might wear out more quickly than they would if you only ride in nice weather. In that case, replace the derailleurs when they wear out.