Squeak
I don’t know what you are basing your theory about non religous people going to schools classified as “yeshivish” due to the higher quality education. Even if you are right unless we assume they do not marry into the yeshivish community, they still contribute to the growth rate. As far as the grwoth rate is concerned ot makes no difference whether it is due to natrual growth or “Imigration”.
I would however dispute your contention that there is a wave of children entering the yeshiva system due to the quality of education. If you have any evidince for this I would love to hear it (really).
There are a number of other ways the growth rate has been estimated (granted each with it’s own flaws) such as counting numbers of siblings and extrapoliating from the chareidi growht rate in EY (estiamted at about 5% although it is partially due to immigration).