Remathilis
Legend
Yes. Every race received official fixed/float stats as errata, and changes to what any given power does were well-precedented in errata across 4e's lifespan. There were zero problems playing Essentials-only characters alongside not-at-all-Essentials characters, and with the sole exception of "internal" multiclassing (that is, a Slayer couldn't MC to a different Fighter subclass), Essentials options had 100% miscibility with non-Essentials options and vice-versa. Indeed, optimization for some Essentials classes, like Vampire and Hexblade, depended quite heavily on their ability to pick up "Original" 4e multiclass options, e.g. Vampires wanted a particular Sorcerer power (Flame Spiral). Essentials also added--not removed nor replaced--racial powers, such as Heroic Effort instead of the bonus at-will for humans...which had already been done in Dragon Mag, for dragonborn (Dragonfear as an alternative to Dragon Breath).
Even the "internal" multiclassing thing was addressed (IIRC within six months of Essentials' launch?), where "Weaponmaster" Fighters (aka "original" Fighters) could spend a feat to swap out an Encounter power for Power Attack and (I think?) Slayers could likewise give up a use of Power Attack for a Weaponmaster encounter power.
Nothing--not one thing--that Essentials did to alter the rules themselves was in any way unprecedented. All PHB3 races, for example, were published with flex stats, at least three months before Essentials--and the Changeling, from Eberron, had flex stats even earlier than that.
Essentials is not, and never was, an edition or a revision or any of that. It was just more options for the existing game. If Xanathar's and SCAG weren't a new edition or "revised" game, neither was Essentials.
I'll agree that it wasn't really a replacement as much as soft reboot. They tried to restart the 4e line with both some QoL updates but also some "revisions" that would appeal to people like me who had left D&D for Pathfinder because we weren't all impressed with how 4e had been up to that point. And I know this because in my case, it almost worked. We were in the middle of Pathfinder game at the time, and we were considering playing an Essentials-based game to see if it had improved. The fact the edition started to shutter, and the Next playtest was starting to be discussed nixed that idea. But the fact that we felt we could potentially jump in AT the Essentials line and ignore much of what came before was absolutely something WotC was hoping could/would happen.