If I understand correctly what he/she wants to point about the number of applications will never decrease is as follows. Suppose there were 25,000 people including me wanted to sponsor their parents in 2015. If we follow your method, my chance of getting into 5000 caps is 20% (=5,000/25,000). Now, if we think about next year (January 2016), it is very unlikely that only 20,000 people (=25,000-5,000) will apply for sponsorship. May be many new applicants will be added to the pool i.e. 20,000 (from year 2015 who was not selected) + new applicant who became eligible to sponsor their parents. So, my probability of getting into 5000 caps will most likely decrease if sample size (number of people who wants to sponsor their parents) increases every year