No, they won't be obsolete, they will merely provide you with plenty of options. Once you know each workout like the back of your hand, you can blend moves from different workouts together on your own to make up your own freestyle training. That's what I do.
STS is quite a commitment. Sometimes you just know that there's too much going on socially, professionally or with the family for you to continue making that commitment for 3 months, so that's when you shelve STS for a while and drag out GS or S&H or 4DS and do thee workouts for a couple of weeks while you survive the busyness of the rest of your life.
They are not lacking in effectiveness either. I believe Cathe's DVDs help you achieve what they are designed to achieve: either greater muscle endurance or greater strength. They just don't demand the same level of consistent commitment, that's all.
With STS your collection will be enriched, not rendered obsolete.
Clare