Sure, 5th+ levels will make ST spells more viable as you get access to these strong spells like fireball, etc. Especially as cantrips like firebolt will be less useful, only dealing 2d6 damage instead of PnP's 2d10.
Though, warlocks will still be heavily incentivized to use eldritch blast given the ease of hitting due to high ground Advantage.
To be fair, Warlocks are always heavily incentivized to use Eldritch Blast by design. They are more magical ranged fighters than actual spell casters.
While PnP Warlocks won't have the benefit of high-ground advantage, a lot of their spells do revolve around gaining it anyway (Darkness + Devil's Sight, Shadow of Moil, etc), because they are Eldritch Blasters first and foremost.
But it will still remain easy to fail concentration checks, against enemies that can shove you prone or cause AoE damage with special arrows/flasks. Being set on fire will ~equally disrupt conc at level 6 compared to level 4 and I assume the # of enemies with these options won't go down at higher levels...
I agree with you that I think going prone = broken concentration is extremely dumb, and I'm hoping that is a bug vs. intentional (i.e. similar to wizards with Cure Wounds). Unfortunately we can't tell since there hasn't been much communication.
With that said, I can't say the current Larian implementation, as flawed as it is, is imbalanced towards casters because the imbalances swings both ways. If anything, the "meta" of using casters have simply changed if we accept the game for what it is.
I.e. right now, being able to action + bonus-action cast spells of
any level is incredibly broken as it almost doubles your action economy and will scale even more. I.e. you can do weird things like drop a big AOE spell and than Misty Stepping far out of reach to basically remain unhittable.
