Wow! There is just no pleasing some people. In thread after thread there just seems to be a lot of complaining about what 5E doesn't have. When ideas are floated they're all bad. No compromise is acceptable at all, working within the system is intolerable, and the one's complaining don't actually offer ideas of their own.
Makes me wonder if the problem is simply that 5E is not 4E...
It's not that there is no pleasing some people. It's simply that people are bored stiff when people offer
PRATTs that are the very first thing that people suggest when they try to create a warlord in 5e. Especially when it has been pointed out repeatedly in this thread that the Fighter is set up to be a martial DPR character and that is simply not the warlord. It's pretty well known why the fighter fails miserably as a warlord.
Fortunately taking the time to demonstrate exactly how wrong your idea is has ended up with a potential solution.
So, this is what I see so far concerning "Warlording" a Fighter:
Within the constraints of 5E (a system without an extensive "Powers" mechanic), a 5E Warlord would not be able to have as many options for any given build as a 4E Warlord would.
As has been pointed out at length, this is a complete irrelevancy. The number of powers doesn't matter. The fact that the fighter
can not handle the archetype does. Indeed various class features of the fighter actively go so far as to undermine the approach used by the warlord.
The best approach for a Warlord is as a Fighter class Martial Archetype.
The fighter's core features like Second Wind, Action Surge, and Multiple Attacks actively undermine their ability to be a warlord. When you have action surge, second wind, weapon mastery (although this is easily tweakable) you are best off handling any combat yourself. And you personally shine in combat. The warlord makes other people shine.
Actually now I come to think of it, this proposes its own fix.
The L3 ability for Warlord allows you to use Second Wind and Action Surge on one ally instead of you - and to hand out all attacks beyond the first each round as extra attacks for one of your allies.
And now we add new weapon masteries.
Lazy: You may hand out your first attack to one of your allies. If you hand out all your attacks to a spellcaster they may use it to cast any cantrip.
Bravura: You may choose to take Disadvantage against all incoming attacks. If you do so, your allies gain Advantage against all enemies adjacent to you until your next turn.
The Battle Master is already a fairly decent Tactical Warlord within the constraints of 5E.
The Battle Master is a barely acceptable port of a 4e
Fighter. As a warlord it falls flat, and even as an archetype it only covers a very narrow sliver of the range covered by a 4e Warlord. Fighters are about being great, warlords about making other people great. Unless you use the subclass features to radically invert the fighter and let others in the party use the abilities that make them formidable, as I have indicated, then the fighter not only doesn't cover the archetype of the warlord but is a long long way away from it.
My question is: Why can't one simply convert some of the Inspiring Warlord's Powers from 4E as maneuver options for the Battle Master archetype?
Because your class isn't about your round-to-round tricks. It's about how you approach the world. And the fighter as it stands in no way approaches the world like a warlord. A rogue would make a better base class for a warlord than a fighter does unless you subvert the fighter's abilities as indicated.