When making Darkvision a cantrip, there is a tradeoff. Yes, a Wizard or a Druid or other caster can prepare the cantrip. But there are many good cantrips and fear of missing out. When I pick cantrips there is always one or three cantrips I wish I had enough to include. Meanwhile, a player can trivially use a torch instead of wasting a cantrip. For players who care about Darkvision, for flavor reasons or for stealth, giving up a cantrip for it is fair.
Re exploration pillar. Adventures that rely on artificial restrictions are misguided in the first place. They lack verisimilitude. If a species genuinely features wings, why wouldnt they be able to use them during their background and at level 1? It is important to make certain concepts available from the start − as well as ensure these concepts balance well.
There is zero reason to prevent an underwater adventure at low levels. It is fun, and if everyone is a water species it happens anyway. There is no reason to gatekeep this.
Even an adventure to Sigil can be a low tier.