Circling back to @Hank Kingsley's point about announcing now for '28, I've been thinking that one reason to NOT would be that it'd be a lot of time for the arrows to land and for voters to get fatigued.
That said, there was an Atlantic article on 6/9 talking about how poorly measured so much of the Democratic campaigns outreach efforts have been, and the TL:DR on it is that what worked in 2016 appears to have really diminishing returns in 2018 and very little impact in 2024. Dem campaign subcontractors and policy groups don't like to talk about this because it undermines their value when trying to solicit donations.
At the end of the article, a source made the point that the GOP is far better at creating in between election engagement and that's helped neuter Dem campaign efforts (i.e., if you're a Rogen listener and you've heard from a number of GOP officials, or comedians mocking incumbent Biden, etc., over the past couple of years, a personalized snail mail letter or really good social media campaign in the 60 days prior to the election isn't having much impact).
Another interesting finding was that door knocking might actually be improving your opponents chances, i.e., voters in high engagement races are already saturated with outreach and are displeased at being bothered in person.
Anyway, if you follow that logic, announcing now gives you the opportunity to build out your coalition more organically and follow the GOP playbook of making it more of a cultural (or for @troph and fellow Greg Casar lovers, economic) movement.