Yeah, me too. I'd like to have as few classes as possible, makes the core game more elegant. 4E had a few classes too many.
Give us more themes (subclasses) instead. I could even live with the 4 base classes alone and the more classic extended classes (barbarian, bard, druid, ranger, etc...) as themes (subclasses) of the 4 base classes.
-YRUSirius
Not trying to come down on you, YRUSirius. Personally, I don't care about elegant. I care about play at the tabletop, not how aesthetically and philosophically pleasing the architecture of the rules is. I totally understand the feeling, though. I used to be that way myself. Nowadays, I just want the rules to play fast, play well, prep fast, prep easy, prep well, and do all that for both players and DMs. I've got too many "beautiful" or "elegant" games collecting dust on my shelf or taking up space on my hard drive to need another. I want to
play 5e, and play the yotz out of it.
Of course, if it plays great
and is elegant...well that's a bonus.

I like elegance, but if the designers have to choose between elegance and play...choose play every time, IMO.
Now, the particular question of demoting old classes to themes....I'm highly in favor of it when its appropriate and effective. I could see scenarios where it would work out for almost all the "second tier" classes people have suggested on this thread and others. However, I couldn't say which ones without knowing more about where all the different mechanical effects and weights.
I find the idea of things like a Wizard(assassin) particularly intriguing, and I think it might reduce the impulse to multiclass as madly as players did in 3.5. I especially like the idea that you could keep some your theme-based abilities rolling right along, even if you multiclass to gain other mechanical advantages. So you start as a Fighter(Slayer), but multiclass into Wizard. Your Slayer stuff could still progress, even as you get more magical mojo.
Keeping only 4 base classes....again, hard to say without knowing how the mechanics will play out. However, I don't think that sounds like the direction they're heading with this. I haven't come away from things with the feeling that they intend themes to do that much heavy lifting. It could definitely be set up that way, but I wouldn't want to them to do it for any other reason than it plays better that way.