1) they've won their classic race (all are for 3 year olds); not much reason to keep them in training, unless you want to confirm their reputation by winning some class races at 4.
2)Horses (5 years old and over) are fully mature , a real handful and not tractable in racing. They do bulk up, but that's not necessarily going to slow them.
3) If the horse is wanted for jumping, he'll be gelded (see (2)); this is not, as commonly supposed, to save his undercarriage from hitting the fences
4) the stud fees commanded by the likes of Frankel will be high; £100,000 is spoken of, but the reality is an untested sire like him will be £30-£50K, and the fees will rise once he has a successful winning crop. But, he'll be covering three or four mares a day,from mid-February (romantically we talk of Valentine's day being the start). That's a lot of money taken over perhaps 18 breeding seasons ! And the foals fetch money too; one sired by Frankel's sire fetched over £2 million, and it's not unusual to see 500,000 guineas paid for a yearling.