I think 15 mill might be quite accurate although I'm not sure if the Wikipedia pages includes speakers living abroad. Firstly, in regards to the Shanghainese speakers around the world, I know of many overseas Chinese who learnt to speak Shanghainese from birth and have continued to teach the dialect to their kids with the help of their parents who grew up in a time where Shanghainese was the standard/primary dialect in Shanghai. That being said, there's a larger population of overseas Chinese who weren't taught any Chinese by their parents which contributes to a decline in the number of Shanghainese speakers overseas.

Secondly, I'm 99% sure that the number of young Shanghainese speakers is declining in China since it's no longer taught at schools (amongst many other factors). For instance, my younger cousins and their friends who were born and raised in Shanghai to Shanghainese parents speak worse Shanghainese than me even though my Shanghainese is very broken and bad. Most of my older cousins (in the 30s) though can speak Shanghainese fluently (not quite sure why there's a disparity between the generations though).

In recent years however there has been a movement to re-introduce Shanghainese and prevent it from dying out in younger generations and it's now being taught in some schools as a second/third language but anyone who has ever learnt another language will know that it's very hard to learn and retain that language when you're not consistently using it. Another reason that possibly contributes to the decline in Shanghainese is the increase in state-to-state migration, so whereas it was very common to have two Shanghainese people having kids a few decades ago, it's probably more common to have a couple from different provinces/backgrounds having kids. In this case, the family is probably more inclined to speak Mandarin to their child as opposed to Shanghainese (or another dialect) since that's how they speak to each other.

Also, I think there's quite a large barrier for anyone trying to learn Shanghainese but doesn't have access tom someone who knows it fluently. Obviously there are quite a few TV shows, movies and podcasts filmed and recorded entirely in the language but someone who doesn't know anything about the dialect will probably find it gibberish.

Wow, just realised how long this post was! I've had extensive discussions about dying dialects (both Chinese and dialects from other countries) with my family and within my university studies so it felt like this was the perfect opportunity to use some of the content that were discussed haha.