Aldarc
Legend
It's not just about whether the warlock has access to higher level spells but also about when they get access to them. A 2014 Fiend Warlock has access to Fireball at level 5. A 2024 Warlock does not have access to Fireball at level 5; instead, their access to 3rd level spells is delayed to 9th level, which is when most games - by WotC's own accounts! - are coming to a gradual end. As far as the game was concerned, the Warlock was treated as a full caster for purposes of spell progression.The old Warlock is only a Full Caster if the party and DM agree to a short rest. The entire premise that they stand with other Full Casters is dependent on other people.
And the new warlock still has access to the higher level spells. It is a little off giving them only one more invocation but turning Mystic Arcanum into an invocation, but you can still get ahold of any Arcane Spell in the game.