I don't think this is the case. People's income change for better or worse over time. Employment and income and not requirements for N-400. But I agree, unless really need fee reduction it's best not to use it. Why? Because USCIS is fee funded. By using fee reduction, USCIS will have fewer resources to adjudicate other cases. Also, for OP, case may go slower. Anything atypical causes cases to slow down. Also, this means paper filing with slower time to get N-400 NOA.