Let alone that in the last two years (2015 and 2016) numbers of participants are declining and the finish time in all the four major race distances is still growing.'

This is a wrong argument. It is enough if just running in general gains popularity. If you take races as a sample from the running population the sample size does not mean that much - the average/median/percentile performance of the sample (if it is representative) may follow the performance of the whole running population!

This is the case for example if the racing population is a uniform random sample. But it may even happen that nowadays larger proportion of slower runners are going to races - because races are much more advertised/known for the hobby joggers than in the past, while elite runners were there always...