I was having a conversation with a few of my friends about Facebook and how the purpose of Facebook has changed majorly for us. We are born in the mid-90s, so Facebook was our everything when we were in our early teenage years. However, now being closer to 30 than 20, Facebook's role has changed for us. Facebook used to be a place where everything was published - and everything in this scenario means everything. It was the only place for communication in addition to text messages after MSN Messenger lost its glam. So after the revelation of being able to chat via the messenger feature of Facebook (and not only post everything on each other's walls), the messenger feature was in active use. Thus, when Messenger was separating from Facebook, I remember wondering why, but also remember not giving it that much thought. But that was not the case for everyone.
Messenger was originally known as Facebook Chat when it was launched back in 2008. Later on, Messenger has added more and more features making it more and more practical and versatile and thus making it a big competitor to other communication platforms. For example, video calling in most of the countries was added in 2015, and e2e-encryption in 2016.
Between 2019-2020 there were approximately 250 million daily app downloads (WebSiteBuilder), so I think that shows the fact that downloading apps is not a problem. Many do it every day basically just to try out an app for fun and maybe deleting it later the same day. So it feels a bit funny that back then downloading Messenger as a separate app was such a big deal - maybe people just complained for the fun of it. But after reading Mark Zuckerberg's interview, where he explained the reasons behind the movement of Messenger into a separate app, the reasons became very clear and they made sense. Obviously, it is a good choice for the company and the statistics have shown the success. Mark knew, that people would get used to it and would eventually stop questioning - just like the situation is today. The same purpose was behind the purchasing process of WhatsApp and Instagram; getting in control of as much possible and getting as much power as possible. From the company's perspective, that move was really clear and clever - of course, it's better to create a separate app to gain more users.
Facebook Messenger, as a separate app, hit 500 million active monthly users. Messenger still exists to be the second most used communication app by 1.3 billion users (Statista), following WhatsApp which has 2 billion active monthly users. Messenger hit its 1 billion milestone back in 2016 (Statista). From the chart above, you can clearly see that the separation of this app wasn't a mistake, since the increase of downloads has been very stable throughout the years. And for the practical part of it, I think it's easier for people to have a separate app if they wish quick communication rather than opening an app and then first selecting the chat feature. The core purpose of Messenger getting separated from the Facebook app was to benefit the users. The pain would only be short-termed, but results would show the benefits - as it obviously did.