It's not quite a simple as that. Firstly Apple never ask if you want to use iMessage. If you get an iphone and try sending a SMS it routes it to iMessage automatically without asking. I guess you can turn that off - I don't know how. Secondly if you switch your iphone for an Android and other iphone users try to send you iMessages, Apple could easily get back to them saying something like "We've been unable to deliver that message to the iphone that used to be there - would you like to try SMS?" but they don't. The fact this doesn't work in spite of people complaining for years suggests they may have deliberately not bothered as it discourages people switching.