I think you would be surprised. The folks who will pay for a premium experience by using a 3rd party app are likely willing to do the same on another platform.
I can get a lot more value from a good Twitter client than I can from a good Mastodon client.
It is hard to believe such a big social network to migrate overnight, but it has happened before with digg moving to reddit.
As far as I know, neither of those are an issue on Mastodon and therefore the value of 3rd party clients isn't there.
I hate subs, but I might make an exception here.