The main point I'd make is that Essentials classes do not supersede non-essentials classes. A 3.5 Ranger is intended to replace a 3.0 Ranger. I have both the PHB Rogue and an Essentials Thief in the same party.
Yep, and essentially(!) there's nothing that would have kept you from having both a 3.0 ranger and a 3.5 ranger in the same game.
It's true that there were some minor changes to skills, but the conversion document explained which skills were rolled into others, so it wasn't difficult to deal with characters having one or the other skillset.
This isn't to say you couldn't try and use both, I remember the last 3.5 campaign I played in, and one player wanted to use a 3.0 prestige class, the Lasher, that, to my knowledge, had not been ported over to 3.5.
Huh?! Have you been in my campaign?
Even after I had switched everything else to 3.5, I allowed my fighter player to take the lasher prestige class (although I later regretted it, since he turned out to be quite overpowered in the end). I decided on a case-by-case basis what I allowed and what I didn't allow, e.g. when one player wanted to take a feat from the Neverwinter Nights CRPG that didn't exist in the pen & paper rules, I allowed it.
Anyway, my point is: Essentials is just about as compatible with classic 4e as 3.5 was with 3.0, it's just been marketed differently (or to use your words: the 'intention' behind the changes is different).
I've seen comments from players on this forum who stated that they decided to change their games into 'Essentials only' games. I.e. they decided to go against the 'intention' by making the Essentials classes supersede the 'classic' classes.
Likewise there are groups (including mine) who decided not to allow any of the Essential classes. Since every new product release for 4e is going to be 'Essentialized' that's likely to change eventually, but for the moment that's what we (or rather our DMs) decided.