I was relistening to the video in the Original Post.
I am reconsidering my view on the playtest Warlock that has partcaster spell slots.
My first view is, the Warlock is very much a fullcaster. The 2014 Warlock is what a fullcaster looks like when using Short Rest refreshes.
When switching to a Long Rest refresh, to represent the Warlock as if a partcaster is jarring.
Crawford in the video makes a good point. Essentially, the fullcasters have the spell progression take up too much design space in the base class. There isnt enough design space for other kinds of base class features. (Nevermind there isnt enough space for meaningful subclasses.) If the playtest gave the Warlock a fullcaster spell progression, there would be insufficient design space for things like Warlock Invocations and diverse subclasses.
For me, the most important requirement of a fullcaster is the ability to cast slot 9 spells at the highest tier. The playtest Warlock can still cast the most powerful spells via the new Invocations. I can work with this. I agree, ample design space for various kinds of Invocations is central to Warlock concept.
This makes me wonder. Maybe the fullcasters have a design problem. At the higher class levels, the low slots have too much bloat. The bloat can be problematic by lacking ease or pleasantness of options during gameplay − issues like choice overload and so on. But the bloat can also be an annoyance such as spamming the Shield spell.
Maybe we can rethink the fullcasters, or at least doublecheck if we prefer the 2014 fullcaster progression.
Maybe the fullcasters do better to look more like the playtest Warlock?
What if. The low slots bloat slims down dramatically. Instead, there are only a few low slots. All of the extra design space is repurposed for interesting class features, and great subclass choices.
Or maybe, the fullcasters use spell points. Say, the fullcasters have a number of spell points equal to 1 + the class level. (The cost of a spell is its slot. So a Fireball costs 3 points. Wish costs 9 points. One cant cast Fireball until class level 5, or Wish until level 17.) This spell pool refreshes every Long Rest. But. All of the extra design space, is used for things resembling Warlock Invocations. This extra design space lets Bladesingers be awesome at gishy melee combat, lets Evokers do fun stuff.
The Warlock is definitely a fullcaster. Maybe we can doublecheck what a fullcaster can be.