There's a lot you aren't taking into consideration, for example where those users are coming from.

• Threads is just Instagram, it's essentially the same account, so its users were baked in from the start. How many of those Threads users are just people who already used Instagram?
• BlueSky really isn't pulling users from Mastodon, it's pulling them from "X.com", which if nothing else represents a breaking up of old Twitter, which is good for decentralization in general. Any fracturing of social media is good for the fediverse.
• Both Threads and BlueSky are, to some degree, copies of Mastodon in terms of their relationship with federation. Threads uses ActivityPub itself (a win for the fediverse, depending on how you look at it), and BlueSky has their own AT federation protocol, which shows that the ideas behind the fediverse are already winning out. It's entirely possible that, at some point in the future, BlueSky and Mastodon learn to speak to each other using one of those protocols (or a new one), and then the fediverse wins by default.
• Fediverse apps like Misskey are apparently doing great in Japan, which is great for the existence of Japanese artists and the international side of things.

The fact that we're here, right now, discussing this on the fediverse, shows that ActivityPub has come a long way. Bluesky is nothing more than a shallow copy of Mastodon with far less federation and far fewer features.

Mastodon is a bit like Linux, due to its free and open nature. It can be in 3rd place for 20 years, but it'll keep chugging along, improving and growing over time until it snowballs into something truly formidable. Linux doesn't need to be the most popular operating system ecosystem to be great, nor does Mastodon need to be the most popular social media server. Unlike a corporate product it's not just going to die and disappear just because it isn't the most popular of the social media platforms.