With all due respect, I think Reb Aron Teitelbaum meant Monsey as an alternative as well, which does include the masses.
I think Rav Miller zatz”al was referring to a time and for those that the suburbs was/is a route to assimilation. Even Lakewood has expanded, and while those on the outskirts are initially among non-Jewish/irreligious, those communities usually become majority Frum very fast. Not everyone can or should be a nachshon – depending on the neighborhood (or even “block״) moving to they have to be very firmly rooted in their Yiddishkeit – so I definitely see his point. And couples have to be honest with themselves as to the primary reason for their move.
But making it a blanket rule can be very detrimental to the klall. Most Brooklynites aren’t fleeing to the suburbs of secular NY, Long Island or NJ nowadays. There’s more than a subtle difference.