I have recent experience talking to non-techie younger people recently about this very issue and none of them were aware of the security differences of SMS vs. iMessage.
Teenagers know that blue = iPhone and green = non-iPhone/SMS and that blue offers significantly more features and functionality vs SMS (delivered/read receipts, group chats, stickers, rich media, memojis, etc), which is the overwhelming reason why blue is preferred.
Plus the cost. Over here in the EU, back in 2010 (?) SMS was an expensive thing, you would pay dearly for each or have to buy a "50/100/500 SMS package" or similar.
So lowering the cost of 3G made it more economical if your friends had iPhones as now you could spend €20/month for '1GB' (which was mostly iMessage & web browsing at the time) and avoid spending that simply on SMS. (excuse the price inaccuracies if any, it's been a while since I had that iPhone 3GS)
Back in 2010 iPhones were expensive status symbols in the EU, approximately nobody bought them to save money on SMS. In some markets the really heavy texters bought Blackberries for a while just for BBM, but that got killed pretty quickly by Whatsapp.
Teenagers know that blue = iPhone and green = non-iPhone/SMS and that blue offers significantly more features and functionality vs SMS (delivered/read receipts, group chats, stickers, rich media, memojis, etc), which is the overwhelming reason why blue is preferred.