You say the game plays well for optimized characters at this lv - but then disparage the melee fighters for optimizing??
Disparaging optimization? Not quite. I'm saying that without those strategies, the class becomes worthless in comparison to their caster counter parts. 5e is decidedly bad at allowing front-liners to BE front-liners without significantly going out of their way to do so. If your player decides he wants to build a straight fighter and stick him on the front line, at most he will have 3 AC more than the rogue in studded leather. His magical friends will be busy casting Shield for +5 AC when at risk, boosting their AC to an equivalent--if not higher--rating.
When it comes to casters, you could skip every form of optimization. You could play a warlock who never learns Eldritch blast (or any of its overpowered alterations), and still come out fine with a fireball later. You can play a Ranger who spent his feats on heavy armor, and unless his stats are all -1 modifiers, he will still perform at a sufficient level, and far from the combat.
The sorcerer in our group is brand new, and didn't know what meta-magic was until 7th level. Nor did he add spellcasting modifiers to his attack rolls. He still contributed a significant portion of our groups damage, despite missing every other spell. Eventually, I realized what was wrong (I wasn't there during character creation, and our DM is trying his hand at DnD for the first time. I would have corrected the issues sooner had the sorcerer not been at the other side of the table sitting next to the PC in our group that DMs a different 5e campaign.)
Long story short, if you can cast a blast, you're set on combat efficiency until legendary creatures start popping up. If you're keen to the blade, it's like preparing to raid; potions? check. Enchantments? Check. Perfect ability rotation and defensive skills so I don't get gimped by all of the advantage rolls that come with being surrounded? Check.
The one exception I can think of would be versatile range/melee mix of fighters/barbs/paladins fighting in formation. Paladins might as well be renamed to "Holy Spell Blade". While barbarians can make the cut as melee DPS, their utility as a front liner is hampered without some form of CC.
On the complete other end of the spectrum, a full group of ranged damage dealers could probably make out just fine in most instances, using only the basics of each class. Repelling Eldritch Blast can push 20 feet, Rangers have all sorts of lock down, Wizards have oodles of AoE, Sorcerers can burn a threat down before he's even realized combat is happened, and Druids/Clerics could function as life support machines during an open-heart surgery.
So yes, the game can play amazingly well for optimized characters at 11+, but melee fighters get the short end of the stick when unoptimized.