Sea The Stars has really nothing left to prove in his 3 Y O season but would’ve had everything to prove at 4
This is exactly my thinking. I am in the ‘nothing to prove’ camp in so much as the BC classic was a lose-lose proposition for the horse right now. STS already is proven as a superstar 3yo, a victory at SA would merely seal the deal that STS is a "superstar" 3yo. A flop would have a great impact on his rep however. It would rub off that brilliant superstar lustre the horse has right now.
I would have loved to see him return as a 4yo too, but £60m is a lot of money in anyone’s book. The question you have to ask is would he be worth more as a stallion after winning a KGVI, Arc, et al, as a 4yo? Suppose that next year’s crop are not all that? I can see all the "he beat nothing" arguements now. Another lose-lose proposition.
The real money is in the breeding side of racing. As seasoned followers of the sport, we all know that. Don’t we?
Erm, the real money, to me, is in the betting ‘n’ winning side of racing 
He’d be worth more in the hearts of millions of racing fans if, let’s say, he reigned supreme at 4 years old. I’m prety sure he’d be worth more as a stallion if he conquered all at 4 too.
Racing is about seeing our favourite horses run against each other. Stud, in my opinion, is a well beaten second.