It might just be an idea but it's not something that EA will ever put in. I'm guessing you never ever had to see someone you lover suffer from and/or die from cancer because if you had, you wouldn't be asking for this in the game. And I can tell you that it's not a pretty sight to watch someone you love have to suffer and try to put up with the pain and all of the side effects of chemo and radiation and watch their hair fall out and getting sick from them. It doesn't matter if EA puts in a way to turn them off, there is no way that EA will ever add them. And if EA was to add this, just think of how much money they probably would lose because people just wouldn't want to buy the EP even though they know that they would be able to turn them off. They only real life illnesses that they probably will add into the game is the cold, the flu, allergies like they did in TS3's Seasons and upset tummies from eating bad food. And I believe the only way for this to happen is if someone makes a mod for it so that if someone wants this, they can add the mod and play like that. That's what mods are for, is for stuff that EA can't, won't or isn't allowed to put in.
Asking for this is just as bad as asking for disabilities, birth defects, cigarettes/cigars, abortions in a game that is meant to be fun. To me, the Sims is a spoof/parody of life. To me, it's a way to allow my Sims to do things that I can never do in real life such as having them be a vampire, witch, fairy, genie, ghost, etc.
Like I said earlier in my first post is that you can use your imagination that your sim is sick with cancer. You can already send them to the hospital with the Get to Work EP and have them get examines, surgeries with the table. So you can pretend that when they get a surgery, that they are getting a cancer tumor taken out. Or if they get sick and throw up from having an upset tummy, you can pretend that they just went through chemo or radiation and now they have gotten ill from getting. See I just pretended that a sim has cancer without it ever being in the game. This game is about using our imagination... ;)