I've often decided on technical decisions with incomplete information. Here's an example of how it works (translated to this analogy):

1) You come up with a working (if expensive or otherwise imperfect) technical path 2) You define a half-dozen other potential (less expensive, more practical) paths 3) You announce the decision 4) You complete due diligence, and take the best path

There are other ways as well. Announcements and plans aren't binding; they occasionally change. You can make an announcement when you're 98% confident you'll do something. You can pivot if it doesn't work out. There are places this doesn't work (e.g. customer promises), but on something like an internal migration, this is a-okay.

If "Wordpress" (which I reluctantly use for our corporate branding site and blog--very carefully managed and controlled) is a better architecture than what Tumblr is using now, what they have now must be truly awful! Wordpress really doesn't scale very well, and you can easily have massive security problems.

Among Tumblr users, the basic incompetence of @staff in constructing a functional website is legendary. I would not be surprised if the backend were far worse than you're supposing.

The funny thing is, the incompetence of @staff is the value proposition for Tumblr, as a user - because Tumblr's backend is a rickety tower of matchsticks and paste and the devs couldn't program themselves out of a wet paper bag, it means that they haven't been able to implement - for instance - algorithmic non-chronological timeline ordering, or competent data harvesting / robomarketing. And the comically broken search tools actually give a reasonable approximation of privacy for discussions. The user experience is firmly stuck in the mid-2000s, when social sites were for communities and discussions instead of data farming.

Don't get me wrong, Tumblr's user experience is also awful - search sucks, tags suck, moderation EXTRA sucks, the website's still overrun by pornographic spambots even after the Great Titty Purge - but any development team competent enough to make real improvements would also be one competent enough to squeeze out what makes Tumblr work.

Generally before a company buys another company, there is some amount of due diligence done beforehand, so I wouldn’t presume there hasn’t been a technical exploration.