The two potential roads seem somewhat equivalent to me:

1. Threads federation is blocked by the main Mastodon instances. A huge user base of non-techies starts using Threads and it dwarfs the rest of the fediverse acting as a singular centralised platform. The fediverse continues to be a techie/ideological anti-corporate community as it is now with a relatively small community in the grand scheme of things.
2. Threads federated with some of the big Mastodon instances. Fediverse instances outside of Threads get a large amount of growth as people see the extra content available in this larger federated environment. Growth of Threads still outpaces all other fediverse instances combined. Meta then carries out some form of EEE tactics and some large chunk of the userbase of the non-Threads instances switch to Threads. The techie/anti-corporate community continues to use fediverse instances without any interaction with Threads.

Both scenarios end in a large centralised platform run by Meta and a small community that want to avoid a corporate platform.

I think it’s also wise to separate the effect of large corporate instances in the fediverse between effects on Mastodon (where users follow users) vs Lemmy/Kbin (where users follow communities). In the case of Mastodon, the effects of EEE tactics will be strong due to a more powerful network effect because it’s important that a particular person is on the same platform as you (i.e. this is a similar situation to XMPP and gchat). In contrast, you just need some people to participate in a Lemmy/Kbin community to make it worth joining, but it doesn’t matter exactly who, meaning that membership can be small and sparse but the community still has a meaningful existence (i.e like niche forums).