I believe there's a third facet, related but distinct from Steve's point about promoting sufficient counter-play: variance-reduction.
Consider for a minute the repercussions of a completely unrestricted format: it would be easy to construct a deck that goldfishes a first-turn win a very high (>95%) percentage of the time. The obvious avenue that comes to mind is a storm kill, but in truth in a format with access to 4 Black Lotus, 20 Moxen, 4 Demonic Tutors, 4 Ancestral Recalls, etc. there are a plethora of valid strategies that consistently win on the first turn.
Of course, Vintage is not a single-player game, and the metagame would adapt to the need to disrupt the opponent when on the draw. And Vintage has access to a surprisingly large amount of tools for doing so, many currently underused: free counterspells are one option, but we also have Gemstone Caverns, Leyline of Anticipation, Chancellor of the Annex, etc.
The result would not, I believe, be a format that lacks deck diversity. Nor would it necessarily be an uninteresting format. It would be very different, and very high-variance. Like in poker, mulligan decisions and the die roll would be critical (even moreso than in Vintage, and that's saying something.) I don't think a best-of-3 match structure would make any sense in such a format: to be skill-testing a match would need to be best-of-N for larger N, and don't believe the claims that a tournament so structured would be skill-less or unfun (just as you shouldn't believe claims that poker is skill-less or unfun.)
Put differently, I've always held that the concept of "fundamental turn," often used to analyze Standard and other slow formats, is not fully applicable to Vintage, where the turn by which a game is strategically decided can vary tremendously depending on matchup, and decks can surge to victory after falling behind for dozens of turns. Still, as a rule of thumb, the fewer cards that are restricted, the earlier the fundamental turn. A key function of the restricted list, especially when it comes to archetype-agnostic cards like the Moxen, is to delay the fundamental turn, prolonging games and reducing variance.