I guess what makes the meaning seem obvious to me is that I've never been in a beta test where they delete only character progress but leave your character name and cosmetic choices in the database. Plus it's much safer and simpler to delete and create a new database (plus a restore for converted v1 characters?), which wipes all changes made during a testing period. This is also a procedure they've tested and performed repeatedly during the beta process.
Writing and testing a program to go through nearly a million characters' data to selectively delete pieces (and/or convert to a new data schema) is a lot more effort, bug-prone, and time-intensive. It's the choice of days or weeks instead of about an hour or so. (I seem to remember Yoshi-P said the process of converting v1 characters to v2 would take over a month.)
Oh, and per Yoshi-P at Nico Nico, we also know that there will be an "early access" for all preorders that Legacy will also qualify for. So it's now confirmed Phase 4 -> Early Access -> Release.