Only the remicade is truly a "life-time" thing. Basically it is a chimeric antibody meaning it has part mouse protein in it. Once you stop, your body can recognize the mouse portion of the protein and make antibodies to it, leading to a serious reaction. Humira, Cimzia, and Tysabri(not anti TNF, but similar protein) are humanized proteins. This means they made the protein look to be almost completely human with such a small portion of mouse protein, it is nearly impossible for your body to detect it as foreign. This leads to your being able to start and stop as necessary. If a doctor tells you that you cannot start and stop these medications, they are simply misinformed and should be corrected and told to research the differences in these proteins to properly understand. A lot of doctors don't understand the difference and think what holds true for remicade is also true for these other medications.
Now, as far as needing them for life, that is true. Until a better treatment comes out, once you stop these drugs, within a month or two tops, you will lose their effect and need to start something else. This is sadly unavoidable, so if you stop one of these drugs you do need to move onto another drug for maintaining your crohns.