And I didn't just say, "it's unbalanced because I think so...". I said it was unbalanced in 4e, and the same could result in 5e, based on reasons which I took the time to spell out. Reasons you avoided quoting or responding to.
It's only the optimal warlords, or builds that the warlord was part of, warlord | bard, or warlord | artificer, that things started to be OP. And even then, it wasn't really broken unless you also had other characters optimized to take advantage of it.
Many of this was due to quadratic scaling. Similar to how the wizard was overpowered in 3e.
i.e. the warlord could give everyone +int to hit (i.e. bless). Which was balanced when you had 3-4 int, but int could be raised to 9 or 10, which made it OP. Then, when combo'd with the ranger's ability to keep attacking until he missed you got infinite damage. They eventually scaled back the ranger to only attack 5 times max, but it still makes for a nasty combo.
Another part is that they expected "basic attacks" to be weak, and thus balanced around that. But you could optimize basic attacks quite a bit. Particularly when then they released some classes that specifically boosted "basic attacks". This same issue actually already exists in 5e. Using BM's commander's strike you might trade 1d8+str, for 2d6+str+1d6 at the cost of 1 die and positioning. A balanced trade. But a BM using commander's strike on a high level rogue, and trade 1d8+str, for 12d6+dex, and that's OP.
4e Warlord was a multiplier, but it needed something to multiply. You can do similar in 5e by having a cleric/bard (bless + insperation) and sorcerer (twin haste) with 2 fighters (sharpshooter+crossbow expertise). High damage * high accuracy * high number of attacks = massive DPR.
Played in a "normal-non-char-op" way, and the 4e warlord was balanced, possibly even slightly under powered.
But, balance is all in the numbers and those can easily be tweaked. Just because bless is powerful, does not mean clerics are inherently flawed characters. You could simply reduce bless to +1.
Likewise, just because casters dominated in 3e, does not make wizards inherently OP. You just need to not scale spells by level.
And again, just because warlords gave +10 to hit in 4e, doesn't mean they need to in 5e. Just prevent the same quadratic scaling.