That is not necessarily the case. T-cells definitely do originate in bone marrow, but then circulate throughout the blood and lymph and can infiltrate other tissues. The could be destroyed at other sites, like in the blood, depending upon the mechanism of targeting/destruction. The body could then still be able to produce new T-cells (though if the destroying influence is still present, they would continue to be destroyed). There are ways to target T-cell production in the bone marrow, of course, for people who have certain types of lymphomas and other diseases.