It’s a problem from content moderation standpoint but also an opportunity. Threads is not trying to steal users from Mastodon, they are already orders of magnitude bigger and current crowd would never switch anyway. The other way around is not so certain. If Threads sucks but you can still participate in it without having an account there then Mastodon becomes a very attractive proposition for people who would never consider ActivityPub based platforms before. Defederating mans you’re robbing yourself of opportunity to court those people.

Also, it’s important to note the timing of when Threads became open to the public and where. For months it was unavailable in the EU because of uncertainties related to Digital Services Act, which among other things enforces interoperability on big platforms. Details for existing ones are still being worked on but Threads was the first big one that launched since it came into effect. It’s been speculated that Threads got a green light from the EU commissioners because they promised interoperability early on. It’s quite likely that Meta had no choice but to open itself up and we’re just enjoying fruits of EU not bowing down to American corpos.