I guess I have that .5 in my sig...so lets go with that.
If you had a ranger or a bard in 3.0 you could still play that charecter, it just changed with 3.5 (and probably got better). I mean, that was always true.
Not really the same. If I had a 3.0 ranger, when 3.5 came out, I had to make changes on my character sheet. I have to adjust my hitpoints. My skill points. Some skills were blended into others. Other skills became class features for me. My key class features underwent significant changes.
With Essentials, if I have a ranger, my character sheet remains the same. The existence of a new build - even one that changes 'default assumptions of the system' doesn't affect my existing character. The changes to racial ability scores doesn't affect me - my existing ability scores remain legal choices.
They've added new options without invalidating existing characters. That's the big difference, and why the claim of compatability remains true.
Now, yes, there are some legitimate genuine changes to existing elements - Infernal Wrath, Magic Missile, Melee Training, and various errata that has come before for existing powers, feats, etc.
But I think it has been on a smaller scale. I think that few have changed things as fundamentally as some of the class changes and spell changes in 3.0 to 3.5.
In a related vein, some options you didn't take may now get better. For me, no big deal, but for other people this matters. With 3.5 half elves became arguably the best bards, now maybe it will be eladrins. Again, I don't care, but some optimizers our there might.
Sure, but how is this different than... any supplement, ever? By this logic, every single book released for the game is a new 'edition' because some options might be better than others? I don't particularly buy that line of thought.
As for your last point, someone may actually do the breakdown. I don't think it is that far appart. Each was dozens and dozens of little changes and a few big ones. Of course, the approach to phasing in these changes in different, as I noted in my first post.
I think it is a lot farther than you believe. There just haven't been that many 'big ones' that had as deep an impact on characters. There have been some, but a change to, say, one paragon path or one fighter build, doesn't feel nearly as extensive as a change to a base class like ranger. There have been a good number of powers with errata - but not nearly as extreme the changes to spells.
I mean, spells changed levels, class lists, names, and many underwent much more fundamental revision. Changes in buff duration, haste, polymorph spells - those affected entire styles of play. As compared to powers that... had some numbers shifted up or down, or a limit placed on how many attacks you could get. And more than that - most classes in 3.5 drew on the same spell list, so changing those spells affected the bulk of spell-casting classes. Since powers are unique to each class in 4E, any given change to a power applies to a much smaller number of characters.
We've had a couple that are wider reaching, such as Tiefling's Infernal Wrath.
But like I said - a Tiefling with PHB Infernal Wrath, alongside one with Updated Infernal Wrath, won't feel nearly as different as someone with 3.0 Haste vs 3.5 Haste.