Doing my best to destroy as much as I can :-) though, it is not self-evident that comparing international conventions is a 100 % easy thing to do, as you perhaps have to take a look at what kind of people attended. Perhaps in 73 there were many visitors from neighbouring countries like Germany, Austria, Switzerland, France - and since the distance was not great, many from these countries were among the ones baptised. Perhaps there were not that many international conventions. Today, perhaps JWs to a larges extent are hand picked, perhaps more of the visitors came from USA or more distant countries, perhaps fewer of them were among the ones baptised because they were elder - who knows? So in order to ensure the figures are comparable, a wide array of investigations would have to be made on the attendance to make sure they REALLY are comparable.
Baptisms worldwide are far higher today than they were even in 73, so I - regretfully - don't buy each and every interpretation of the figures.