Well it might have played differently. If Facebook hadn't bought it then Facebook would have competed against it and that's not an easy fight. Look at Snapchat.
Also, as it was made that Instagram only had 7 employees when they were purchased, that's actually one of major reasons why the acquisition worked so well. 7 employees for an application of that scale is not "swift and agile", it's criminally understaffed. After the Facebook acquisition, Facebook was able to easily redirect some of their hires to work on Instagram which allowed Instagram's original workforce to actually be able to worry about things other than just keeping their heads above water. It's definitely not ridiculous to credit some of Instagram's success to the acquisition itself.
FB was pretty slow to monetize Instagram, and they still had hired 100s of people pretty quickly after the acquisition.
Before facebook bought Whatsapp they only had 5 employees. But they had the best engineers for the task at hand. In fact they’re famous for handling 2 million TCP connections on a single server due to much of their engineering decisions (including choice of tech stack). And it’s not like they weren’t generating revenue. Before facebook acquired them, their $1 fee earned them $1billion the first 9 months.
And additional features are going to add massive complexity and decrease your ability to scale relatively quickly. Sure, with the right engineering you might be able to run a relatively light operation if you have a feature-set well thought out, but as soon as you want to add any other thing-a-mo-bob to the operation it gets much harder.
Whatsapp also only had a couple hundred million users back in 2014. So they’d have no way to make a billion even if they charged everyone. Which they didn’t. They barely charged anyone.