That happened with Dota 2 as well.
I have been wanting a game like this for years but I have my reservations about this leaning more into the Dota elements than shooter elements (a game I liked did that for its sequel and it was less fun compared to the original)
Hoping to have some free time (and peace) to try it out soon.
From personal experience, i can tell you that DotA was unimaginably popular for a Warcraft 3 mod well before dota 2 came out. Practically 8/10 games on the custom games list for warcraft 3 was dota, and about ten times as many was played in private garena lobbies with their ranking systems. My middle school self was so fucking excited for dota 2 that i spent 30 hard earned dollars on an invite that was given for free to some users in the 2011 steam winter sale event. At the time, the only two alternatives were HoN (which was a pretty shit clone tbh) and LoL, which even back than was considered a watered down version for babbies.
DotA's growth was not because of any kind of crazy viral marketing, it was simply because it was a one to one copy of the original, spearheaded by the same person who managed the original mod for so many years before. Add to that a general disdain for Pendragon, (due to suddenly shutting down the biggest DotA fansite and replacing it with a message practically saying "party's over idiots, come play LoL instead") and the only choice left was DotA 2.
I think the biggest evidence i have of this is the fact that the game peaked in 2015 to 2016, which is when the last patches for the original warcraft 3 mod was released, essentially forcing the last few stragglers to make their eventual choice of ASSFAGGOTS.
The inherent problem with DotA is that you need to put so fucking many hours into the game to even grasp the basics, and even in unranked baby matches people will shittalk you for not knowing how to pull creeps or know how to refill bottles when teleporting. Generally not a great time investment for anyone looking to get into a new game, and it shows. Ever since its peak in 2016, the game has been on a very slow, but noticable decline. The people who play are diehard fans who will stay to the bitter end, but the people getting in are fewer and fewer.
I really really wish that the game would be able to see some kind of renaissance, but I don't see it happening. Half the community wants new patches constantly, and the other half wants nothing to change, and nobody dares to look in because you need a postdoc to even get into a game.